2007:277 CIV MASTER’S THESIS
Alternative Methods for Heat Stress Relief
Stefan Lindqvist Jonas Holmgren
MASTER OF SCIENCE PROGRAMME Mechanical Engineering
Luleå University of Technology Department of Applied Physics and Mechanical Engineering Division of Manufacturing Systems Engineering
2007:277 CIV • ISSN: 1402 - 1617 • ISRN: LTU - EX - - 07/277 - - SE Universitetstryckeriet, Luleå Alternative methods for heat stress relief Preface As a final element in the Masters of Science degree at Luleå University of Technology a master thesis is conducted by the student. This is usually carried out in cooperation with the responsible tutor and a company with a connection to the student’s direction of interest. The master thesis lasts one term and can be performed at the University or on site at the company. The student attains a degree in chosen direction of interests after passed verbal presentation and written report both at the company and at the University.
We have chosen to perform our master thesis at Ferruform AB in Luleå at the department for axle production. The thesis work has been carried out during June to December 2007 and involved investigations of alternative methods for heat stress relief. The work has been instructive and interesting and a good basis for future work within the manufacturing industry has been attained.
We would like to thank all involved personnel that have been helpful during our work. Great thanks go to our supervisor at Ferruform, Peter Lundkvist, for guidance and advice to our thoughts during the work period. We also say thanks to our tutor and examiner at Luleå University of Technology, Hans Engström.
Luleå 14th of December 2007
______Jonas Holmgren
______Stefan Lindqvist
Alternative methods for heat stress relief Abstract Ferruform AB in Luleå is a manufacturer of chassi components for Scania’s truck production. At the department for Axle production sheet metal is processed to produce rear axle housings. The main processes include shearing, forming, welding and machining. High amount of residual stresses are built up in the material because of the extensive weld operations. Residual stresses are reduced with a very time and energy consuming heat treatment method called heat stress relief.
The purpose with this thesis was to investigate alternative methods for heat stress relief and then suggest the best solution for Ferruform.
Our main tasks in this thesis has been literature studies, investigation of prior work, market analysis, contact and visits from suppliers, production tests and weekly meetings with our supervisor. Finally we have presented the thesis verbally at the company and at Luleå University of Technology. A written report has also been made.
Two alternative methods for heat stress relief has been found and further investigated. Tests have been made regarding these two methods for stress relief. One test with a method called vibratory stress relief and one test with a simulated bend-straightening operation.
Layouts have been created to easier visualize the space required for the different methods. Seen from the layouts is that both methods requires less space than the heat stress relief oven. Single piece flow is achieved with bend-straightening and batched flow is achieved with vibratory stress relief.
Bend-straightening provides single batch flow, low cycle times and is utilized by a competitor, therefore it is recommended as a primary option to heat stress relief at Ferruform.
However, verification tests have to be made in order to evaluate the effects of the two different alternative methods. We have suggested that a fatigue test with spectrum loads should be carried out in order to get a reliable result.
Also a third method is taken into account in this thesis, hot formed banjo halves. This is not an option for Ferruform but may be an alternative to Scania Sao Paolo in Brazil.
Alternative methods for heat stress relief Sammanfattning Ferruform AB i Luleå tillverkar chassikomponenter till Scanias lastbilsproduktion. Vid avdelningen för Axelproduktion utförs förädling av plåt till färdig bakaxelbrygga. De processer som utförs är huvudsakligen klippning, formning, svetsning, bearbetning. På grund av omfattande svetsoperationer byggs svetsegenspänningar in i materialet som kan orsaka geometriavvikelser och minskad livslängd. I nuläget utlöses dessa svetsegenspänningar med en mycket tids och energikrävande metod kallad avspänningsglödgning
Syftet med examensarbetet var att undersöka vilka alternativa metoder till Avspänningsglödgning som finns idag samt rekommendera den bästa lösningen utifrån Ferruform’s perspektiv.
Våra huvudsakliga arbetsuppgifter har inneburit en rad olika moment som innefattat litteraturstudier, undersökning av tidigare arbeten, marknadsundersökningar, kontakt och besök av leverantörer, tester i produktion samt återkommande veckomöten med handledare. Slutligen har vi redovisat muntligt på företaget och vid Luleå tekniska universitet samt skrivit en rapport.
Två potentiella alternativa metoder för avspänningsglödgning har undersökts närmare. Tester har utförts med dessa två alternativa metoder till avspänningsglödgning. Ett test utfördes med en metod kallad vibrationsavspänning och ett test med en simulerad riktningsoperation.
Layouter har tagits fram för att lättare kunna visualisera det utrymme metoderna upptar. Man kan se från layouterna att båda metoderna upptar mindre utrymme än den nuvarande avspänningsglödgningen. Enstycksflöde fås med riktningsmetoden medan ett batchflöde fås med vibrationsavspänning.
På grund av att riktningsmetoden ger enstycksflöde, låg cykeltid samt används av konkurrent gör det den till den metod som rekommenderas i första hand på Ferruform.
Dock måste ett verifikationstest göras för att kunna utvärdera effekterna av de två alternativa metoderna. Vi har föreslagit att ett utmattningstest med varierande lastpåkänning bör utföras för att kunna erhålla ett så tillförlitligt resultat som möjligt.
Även en tredje alternativ metod har undersökts i detta arbete, varmformade banjohalvor. Detta är inte en metod som lämpar sig för Ferruform men kan vara ett alternativ för Scania Sao Paolo i Brasilien.
Alternative methods for heat stress relief Nomenclature
Relaxation Reduction of residual stresses Jolting Forging operation Dislocation A linear crystalline defect around which there is atomic misalignment
Banjo Part of rear axle housing Wedge Reinforcement part in rear axle housing RT Room temperature
Alternative methods for heat stress relief Table of Contents 1 INTRODUCTION ...... 1 1.1 COMPANY BACKGROUND...... 1 1.2 CURRENT SITUATION ...... 2 1.3 PROJECT DESCRIPTION...... 2 1.4 GOAL ...... 3 1.5 LIMITATIONS...... 3 2 PLANNING ...... 4 3 THEORY ...... 5 3.1 WELDING...... 5 3.1.1 MIG/MAG- welding, Gas metal arc welding...... 5 3.1.2 Resistance welding...... 6 3.1.3 Weld zones...... 7 3.2 RESIDUAL STRESSES CAUSED BY WELDING ...... 8 3.3 FATIGUE ...... 9 3.3.1 Causes...... 10 3.4 FATIGUE STRENGTH...... 10 3.5 FACTORS AFFECTING FATIGUE ...... 11 3.5.1 Type of load...... 11 3.5.2 Range of stress...... 11 3.5.3 Mean stress...... 11 3.5.4 Construction design ...... 11 3.5.5 Environment ...... 11 3.5.6 Surface treatment...... 11 3.6 HOW WELDING AFFECTS THE FATIGUE STRENGTH ...... 12 3.7 HEAT STRESS RELIEF ...... 12 3.7.1 Reheat cracking caused by heat stress relief ...... 13 4 PROCESS DESCRIPTION REAR AXLE HOUSINGS...... 14 5 METHOD SELECTION PROCESS...... 15 6 TEMPERATURE DEPENDENCE DURING FORMING ...... 16 6.1 HOT FORMING ...... 16 6.1.1 Benefits...... 16 6.1.2 Disadvantages ...... 16 6.2 COLD FORMING...... 17 6.2.1 Benefits...... 17 6.2.2 Disadvantages ...... 17 6.2.3 Increase of residual stresses leads to:...... 17 6.3 WARM FORMING ...... 18 Benefits compared to cold forming...... 18 Benefits compared to hot forming...... 18 6.4 SPRINGBACK...... 19 6.5 COMPARISON BETWEEN HOT/WARM FORMING AND COLD FORMING OF REAR AXLE HOUSINGS ...... 19 6.5.1 Tension test result ...... 19 7 BEND-STRAIGHTENING ...... 20 7.1 METHOD...... 20 7.2 SUMMARY OF READ REPORTS...... 20 7.2.1 Relaxation of residual stresses through cyclic load ...... 20 7.3 RESULT...... 23 7.4 EXAMPLE OF A BEND-STRAIGHTENING MACHINE...... 24 7.4.1 Automatic straightening of rear axle housings ...... 25 8 STRESS RELIEF BY VIBRATION ...... 26 8.1 HISTORY...... 26 8.2 BACKGROUND...... 26
Alternative methods for heat stress relief 8.3 GENERAL INFO ...... 26 8.4 IMPLEMENTATION OF VIBRATORY STRESS RELIEF...... 27 8.5 THE MECHANISM BEHIND VIBRATORY STRESS RELIEF...... 27 8.6 VIBRATION DURING WELDING ...... 28 8.7 REFLECTIONS...... 28 8.8 DIAGRAM AND PICTURES ...... 29 9 TESTS WITH ALTERNATIVE METHODS FOR HEAT STRESS RELIEF ...... 32 9.1 VIBRATORY STRESS RELIEF TEST...... 32 9.1.1 Description...... 32 9.1.2 Method ...... 32 9.2 BEND-STRAIGHTENING TEST ...... 35 9.2.1 Description...... 35 9.2.2 Method ...... 35 9.3 CONCLUSIONS...... 37 9.3.1 Vibratory stress relief...... 37 9.3.2 Bend straightening...... 37 9.3.3 Verification...... 37 10 TEST WITH BLASTED BANJO HALVES ...... 38 10.1 BACKGROUND...... 38 10.2 PERFORMANCE...... 38 10.3 RESULT...... 39 10.3.1 After Banjo weld...... 39 10.3.2 After welding of cover and ring...... 39 10.3.3 After finish weld ...... 40 10.3.4 After blasting the half time without lances...... 40 10.4 CONCLUSIONS...... 41 10.5 DISCUSSION ...... 41 11 INFLUENCE OF PRODUCTION CHANGE...... 42 11.1 BACKGROUND...... 42 11.2 BEND-STRAIGHTENING ...... 42 11.2.1 Summary ...... 42 11.2.2 Main factors...... 42 11.3 VIBRATORY STRESS RELIEF...... 45 11.3.1 Summary ...... 45 11.3.2 Main factors...... 45 11.4 HOT FORMED BANJO PARTS...... 47 11.4.1 Summary ...... 47 12 DISCUSSION AND CONCLUSION...... 51 13 RECOMMENDATION ...... 52 14 REFERENCES ...... 53 15 APPENDICES ...... 1 Appendix 1. Summary of travel reports Appendix 2. Prior investigation Heat stress relief Appendix 3. Specification of requirements on rear axle housings Appendix 4. Geometry and lifetime requirements
Alternative methods for heat stress relief
1 Introduction
1.1 Company background Ferruform AB is a subsidiary to Scania CV AB and is stationed in Luleå. They have been manufacturing quality products since 1967 and are the second biggest manufacturing company in Luleå beside SSAB. The number of employee’s year 2007 was around 638 and the turnover exceeded 1000 million SEK. The proportion white collar was 97 and 33 women. The plant surface has a total amount of 45 000m2, annual sheet metal consumption is 50 000 tonnes which is 200 tonnes daily. Ferruform AB is certified according to quality management system ISO 9001 and environmental standard ISO 14001. The production today is mainly chassi components for buses and trucks and the lead products are rear axle housings, cross- and side beams and bumpers. The manufacturing processes are technically advanced and following processes are used:
• Blank manufacturing - Shearing with power shear machine and laser machine
• Forming - Automated 2- and 3-press line
• Roll forming - Automated roll forming of side beams
• Welding - Manually and robot welding with MIG/MAG - Laser and friction welding
• Machining - Machine cutting with multiple purpose machines
• Surface treatment - Zn/Mn fosfating - ED- and powder coating
The delivery volumes are divided as follows Scania: 93 %, Volvo 2,8 %, MAN 2,7 % and VBG 1,5%.
Products made in Luleå are transported to Södertälje Sweden, Angers France and Zwolle Netherlands where they are assembled. Rear axle housings are transported to the axle plant in Falun Sweden.
Ferruform AB is currently increasing their production capability to be able to deliver products for the growing demand of trucks and busses. This represents a significant increase in production capability compared to year 2004.
Scania CV AB has developed their own production system called SPS, Scania Production System. Within this system the employees take an active interest in increasing the quality, productivity and cost effectiveness.
1 Alternative methods for heat stress relief
1.2 Current situation The rear axle housings manufactured at Ferruform AB represents a vital component in a truck chassi. The drive line is mounted to the rear axle housing as well as brakes and wheel suspension. The assembly takes place in Falun, Sweden.
The rear axle housings consist of components manufactured at the factory in Luleå, only some parts are bought. Many operations are necessary to manufacture rear axle housings and high demands are put on every operation.
The components are cut and formed from sheet metal. These together with a number of smaller components are welded together. Heat stress relief is used in order to reduce the residual stresses due to welding.
Afterwards painting and machining operations are performed so that the final geometry and tolerances that is required for assembly is achieved. The rear axle housings are then washed and packed before transportation to customer.
1.3 Project description Ferruform AB is supplier of rear axle housings to Scania CV AB and stands for 90 % of the European market. A number of different components are welded together and mounted into the frame of the truck. Residual stresses are built into the material during welding and this will effect geometry and in some extent life time.
Current method used for relieving stresses is heat stress relief. This method is time consuming and a lot of space and energy is required for the oven. The capacity of the oven will be insufficient due to increased volumes the following years.
The housings also need shot blasting after heat treatment. Scania has got a similar factory as Ferruform stationed in Sao Paolo, Brazil. The plant in Brazil manufactures housings with similar equipment as Ferruform AB and also uses heat stress relief. They are currently planning on investing in a new oven and if an alternative method could be used the result of this project will be important information for Scania Sao Paulo in Brazil.
2 Alternative methods for heat stress relief
1.4 GOAL
PRIMARY GOAL
• Propose the best alternative method for heat stress relief
SECONDARY GOALS
• Summarize facts from former investigations and experiences • Investigate manufacturing methods used by competitors • Define specification of requirements concerning geometry and life span • Map and investigate alternative methods technically and economically • Perform test and verification of method • Propose solution
1.5 Limitations
• Focus is aimed at the technical investigation • No investigation regarding other heat stress relief ovens • No investigation in possibilities for optimizing the current heat stress relief oven
3 Alternative methods for heat stress relief
2 Planning Before the project started a plan was made for the working period, see below.
Table 1. Plan Date Week To do Milestones 04-jun 23 Introduction, time and project plan Have a finished time- and project plan 11-jun 24 Theory 18-jun 25 Theory/Flowchart Basic understanding of the projects Summary of former projects, investigate 25-jun 26 concurrent manufacturers methods Understanding of former workװ02-jul 27 09-jul 28 Define specification of requirements Specify requirements 16-jul 29 vacation 23-jul 30 vacation 30-jul 31 vacation 06-aug 32 vacation 13-aug 33 Investigate alternative methods װ20-aug 34 װ27-aug 35 װ03-sep 36 װ10-sep 37 װ17-sep 38 Find new methodsװ24-sep 39 01-okt 40 Put forward solutions װ08-okt 41 Summarize new solutionװ15-okt 42 22-okt 43 Ev. presentation FF Ev. presentation of results 29-okt 44 Ev. presentation FF Ev. presentation of results 05-nov 45 Finish report ( (20th wװ12-nov 46 19-nov 47 Reserve time Finished report for hand in Reserve time/ Report to tutor Hans 26-nov 48 Engström 03-dec 49 Reserve time/ ev. completing time Finished report for print 10-dec 50 PowerPoint 14-dec 50 Presentation at university
4 Alternative methods for heat stress relief 3 Theory
3.1 Welding Welding is a very common method used for joining metals and is widely utilized within the manufacturing industry. Welding can be done with many different methods but common is that the joints are heated up and melted together. Filler material in form of wire or powder is used in order to achieve a more heaped weld joint. The energy required to melt the joints can come from a gas flame (acetylene and oxygen), an electric arc, electrical resistance electrodes, laser and friction. The most commonly used weld method when robotic welding is MIG/MAG and resistance welding.
There are risks for complications during welding. If supplied energy is too low in relation to thickness of the surrounding material cooling rate is rapid and martensite can be formed. The material becomes harder and more brittle if martensite forms. Hardness after welding should not exceed 350 HV [1]. The risk for complications can be determined by calculating the steel coal equivalence, Ec [2]. See equation 1.
Mn Cr + Mo + V Cu + Ni E = C + + + (1) c 6 5 15
The term easy welded steel has an Ec< 0,33. Steel with a higher Ec value between 0,33 and 0,41 has a limited weldability, warming of the work piece is possibly needed in order to decrease the cooling-rate. Steel with Ec> 0,41 is hard to weld and special tricks may be needed when welding e.g. thick sheets [1,2]. With higher Ec value follows higher risk for martensite to form in HAZ.
3.1.1 MIG/MAG- welding, Gas metal arc welding This is an electrical metal arc welding technique with shielding gas that protects the arc against air, see Figure 1.
Figure 1: MIG/MAG-welding [3]
5 Alternative methods for heat stress relief The characteristic of this method is that direct current is used. The negative pole is connected to the work piece, i.e. the work piece is grounded. The additive is in form of a wire and it is fed automatically and has also the function of an electrode. When the wire contacts the work piece an arc is established which melts the wire and the base material together. MIG/MAG- welding is performed in a shielded atmosphere by the shielding gas that always surrounds the arc.
Shielding gas is usually carbon dioxide, CO2 or a mixed gas with 80% argon and 20% carbon dioxide when MAG welding [3]. The argon/carbon dioxide mix is preferred due to a higher tensile strength and a higher toughness is attained. Also a smoother joint, less weld residue and a softer arc is attained. But the mix is more expensive compared to pure CO2.
There is a minor difference between MIG and MAG-welding. MIG-welding is mainly used for welding of alumina, copper, nickel or alloys of these. The shielding gas is argon or a mix of argon and helium. Materials that are welded with MAG are generally unalloyed and low alloyed common construction steel.
It is relative easy to automate MIG/MAG-welding. There are few stops needed for change of electrode when the additive wire is contained in barrels up to around 200 kg each. There is also no need to remove slag from the joint.
3.1.2 Resistance welding This weld method is commonly used within the automotive industry and then often in form of spot welding. The method is mainly used when welding thin metal sheets. Work pieces are joined with one or more spot welds or seams and the ends overlap each other, see Figure 2.
Figure 2: Resistance welding [3]
Spot welding is performed with two water-cooled stick shaped electrodes that presses the sheets together. The machine senses when the right pressure is obtained and switches on the current. After a certain period of time an adequate temperature is reached and the weld occur between the electrodes. The current is then switched off and the weld cools down after the pressure is relieved on the sheets. The joint is completed.
6 Alternative methods for heat stress relief Resistance welding is normally used for welding of common construction steel with an overlap joint and up to 3mm thick metal sheets [3]. It is also possible to weld some copper- alloys, alumina, nickel and zinc plates. Spot welding uses high current combined with short warming time. Therefore the heat energy is used effectively. Very small amount of the heat is lost to the surroundings. Because of that there is many advantages compared to other weld methods.
• Modest deformation of the work piece, heat energy is limited to the spot weld • Very high production rate • Easy to automate, high repeatability when mass producing • Energy effective and environmental friendly • Fast spot welding • No additive required • Low demand of education • Environmental friendly compared to metal arc welding
Within the automotive industry these electrodes are commonly fit to an industrial robot so that the welding of e.g. body components is automated. An ordinary car can have up to 5000 spot welds.
3.1.3 Weld zones When welding a material extensive heat causes the material to form different zones with different properties, see Figure 3.
Figure 3: Weld zones [3]
The heat affected zone, HAZ has a range from the melt boundary to the boundary for material unaffected by the heat. This heat affected zone has due to extensive heating followed by quick cooling experienced a structural change which causes a more brittle material. This is why cracks usually occur right next to the weld joint, i.e. in the HAZ [4].
The weld joint properties are mainly determined by additive, base material and weld method. The properties of the heat affected zone are primarily determined by the composition of the base material and supplied energy during welding. The supplied energy can be calculated with equation 2 [1, 2].
7 Alternative methods for heat stress relief U × I q = × µ (KJ/mm) (2) v ×1000
U = Voltage in volt (V) I = Current in ampere (A) v = Weld speed in mm/sec µ = Efficiency (approx. 90 % for MIG/MAG)
Due to the fact that supplied energy is of the out most importance for the toughness in HAZ the supplied energy should not exceed 2,0 kJ/mm if toughness down to a temperature of -60ºC is required [2].
3.2 Residual stresses caused by welding Residual stresses occur in the material when metal materials are machined or affected thermally. Especially welding causes these tensile and compression strengths within the material and this are due to large temperature gradients. The residual stresses can locally reach the yield stress, σs of the material which arise due to differences in expansion and shrinkage during welding. Tensile residual stresses arise along the weld joint and compressive residual stresses occur across the weld joint. Tensile stresses arise as a result of a will for the weld joint to shrink rapidly in longitudinal direction but the surrounding material does not allow this shrinking to happen. The laws of mechanics state that the sum of all stresses in a cross-section should be zero when no outer force affects a work piece. This is why compression stresses arise in transversal direction, see Figure 4.
Figure 4: Residual stresses [2]
There are different types of residual stresses in a material and they are usually divided in classes depending on how big influence they have on the strength of a material. There are three classes:
• Macro stresses that reaches over several grains • Micro stresses that reaches over few grains • Micro stresses within one grain
Macro stresses have the biggest influence on the fatigue strength while micro stresses are important for the stability of macro-stresses [2].
8 Alternative methods for heat stress relief
Not only temperature gradients cause residual stresses, three other factors are also important:
• Joint geometry • Degree of restraint • Weld order when welding multiple strings
The joint geometry has a great influence in how much residual stresses that are built up since the residual stresses in the material varies with depth. The degree of restraint has also a large influence on how much residual stresses that are formed. No thermal movements are allowed if the material is restrained very hard which leads to a higher amount of residual stresses in the material.
The second string will anneal the first if weld strings are laid on top of each other. This will reduce the amount of residual stresses. Although in most cases in our days automated workshops only one string is laid when welding. This results in high energy input which leads to a large HAZ. The risk for brittle fracture increases.
3.3 Fatigue Fatigue is a fault that occurs in a material that is exposed for cyclic loading such as bridges, aircrafts, trucks and cars. Figure 5(a) illustrates a reversed load cycle where the stresses vary from maximum tension (+) to maximum compression (-). The magnitude is equal. Figure 5(b) shows a repeated load cycle where the tension and the compression stresses are asymmetrical relative to the zero level. Figure 5(c) illustrates a random load cycle where the stresses are randomly varied over time.
Figure 5: Dynamic loading [5]
Rupture can occur after a certain period of time despite of smaller changing loads than the rupture or the yield stress for a static load. Fatigue is a phenomenon that will happen to all metal materials after a long period of time. As much as 90% of all faults in constructions come up as a result of fatigue [5]. Rupture caused by fatigue is sly and can cause disasters as they arise hasty and without warning. Thus
9 Alternative methods for heat stress relief material can rupture like a brittle fracture if it has been exposed for cyclic loading over a long time despite that it is ductile. A material exposed for loads smaller than the fatigue limit is fatigue resistant. The fatigue limit for most steels is between 35% and 60% of the rupture strength [5]. Maintenance of every part of the construction is of the out most importance as most constructions are exposed to fatigue sooner or later. It must be specified when a part has to be replaced before fatigue rupture occurs. This is of course very expensive and cost a fortune all over the world every year.
3.3.1 Causes The causes for fatigue rupture can be divided in three very clear steps:
1. Crack initiation 2. Propagation 3. Rupture
The crack initiation starts in areas with defects or in areas with locally high stress concentration. These cracks usually occur at scratches, threads, sharp edges and dents. The crack grows during propagation and advances towards the interior with every load cycle. The final rupture takes place as soon as the propagation has reached the critical limit. The rupture happens quickly and does always come up perpendicular to the main-stress/elongation direction. It is easy to see if the rupture is caused by fatigue, see Figure 6. The initiation phase is clearly shown at A. Later the crack propagates at B with stripes clearly visible, called beach marks. These prolong until a point C where the material finally ruptures. [2]
A B C
Figure 6: Fatigue rupture [6]
3.4 Fatigue strength Fatigue strength implies the largest stress a material can be exposed to without breaking during a certain number of load cycles. Common steel material has got a fatigue limit at about 2*106 load cycles [2]. If the construction is exposed for smaller loads then the fatigue limit it will resist material fatigue.
10 Alternative methods for heat stress relief
3.5 Factors affecting fatigue1
3.5.1 Type of load How large the supplied force is, direction of the load, at what frequency does the load affect the material, how many load cycles and what stress concentration the material is exposed to.
3.5.2 Range of stress
The range of the stress, σR is the factor that is dimensional. Fatigue strength can be expressed in σR as this factor gives a value of how exposed the construction is for fatigue.
3.5.3 Mean stress
The mean stress, σm affect the fatigue strength. The time before fatigue rupture will decrease if the mean stress on a material increases.
3.5.4 Construction design Design has a big influence on the fatigue life. Every geometrical deviation can lead to high stress concentrations when applying loads. Deviations can take shape in forms of holes, threads, melt ditches and bad geometry transitions. Sharper deviations lead to higher stresses at these areas. When designing a construction it is important to avoid small radius on fillets, diameter changes and other big deviations from the base geometry, see Figure 7.
Figure 7. Larger radius increase fatigue life [5]
3.5.5 Environment Corrosive environment affect fatigue strength because it is easier for crack initiation to occur and therefore fatigue life will decrease.
3.5.6 Surface treatment Small scratches can occur during machining which will affect the fatigue life. It has been observed that polishing surfaces after machining will increase fatigue life considerably. It has also been found that the most effective method for increased fatigue life is to induce compression stresses at the material surface. These induced stresses reduce external tensile stresses when some of them are neutralized by the induced compression stresses. One method for inducing compression stresses is shot peening. Small hard particles, often steel balls with a diameter of between 0,1-1mm with high velocity are shot against the material surface. The resulting deformation has the effect that compression stresses are built in the surface with a depth of approximately half of the diameter of the steel balls, see Figure 8.
1 Reference for above text can be found at [2] and [5].
11 Alternative methods for heat stress relief
Figure 8. Shot peening increase fatigue life [5]
3.6 How welding affects the fatigue strength Welding will have a negative effect on the fatigue strength because of the residual stresses that are induced in the material. There are always a lot of residual stresses in a weld joint. The induced residual stresses affect the crack initiation and the propagation of fatigue cracks. High amount of tensile stresses induced into the material will decrease fatigue life. Tensile stresses have a negative effect on the initiation and the propagation of cracks. Fatigue cracks grow faster through an area with tensile residual stresses than in an area with compression stresses. This is because of compression residual stresses can close cracks which slow down the crack propagation. Crack closure can occur if the applied load only consists of compression stresses and only small tensile stresses affect the area nearby the crack. Tensile residual stresses in the weld joint have to be relaxed properly if a better fatigue life is desired. The relaxation is generally performed with heat stress relief or bend-straightening.
3.7 Heat stress relief As the name indicates this is a method for decreasing the built in stresses in a material by exposing it to heat. These stresses arise in every metal exposed to following operations:
• Plastic deformation by machining as turning, milling, drilling, grinding etc. • Inhomogeneous cooling after high supply of heat such as welding or casting.
The inner stresses can cause the material to twist so desired geometry can not be obtained. Heat stress relief relaxes the material by reheating it to a temperature between 550ºC- 650ºC for steel [7]. The material is held in the heat a certain period of time and is later cooled down in a controlled manner. The temperature and the time are dependent on the steel quality. The selected temperature is a compromise between relaxation of stresses and preserving the yield stress of the material at room temperature. Residual stresses exceed the yield stress of the base material when the material is heated. This causes a plastic deformation so the dislocations can return to its origin and a relaxed material is attained.
12 Alternative methods for heat stress relief This heat treatment is often recommended after welding to reduce the negative effects on fatigue life caused by residual stresses. Heat stress relief is performed after rough machining but before the final finish machining like polishing and shot peening.
Although there is some negative aspects with heat stress relief. The method is very energy and time consuming and the oven requires a lot of space. Another disadvantage is that the toughness and strength in HAZ can drop. That is why heat stress relief should not be done unless it is necessary and there are requirements on performing it. The method is usually used when there are tight tolerances and when risks for brittle fractures exist.
A construction can rarely be completely relaxed with heat stress relief. A constructions tendency to relax is dependent on the complexity of the geometry. Other main factors are degree of restraint and stiffness of the material. Noteworthy is that it is not possible to relax a construction if the geometry is complex. It would take too long time in the heat to reduce the residual stresses which would soften the base material.
Not all steel qualities are suitable for heat treatment. Special consideration has to be taken to type of material, applied heat energy and the complexity of the construction when deciding heating speed, temperature, time in the heat and cooling rate.
3.7.1 Reheat cracking caused by heat stress relief In order to reduce residual stresses the material has to be reheated. This reheating can cause so called reheat cracking. The origin of these reheat cracks is material dependent and they arise mainly in thick low alloyed steels or in hot strength steel. Crack initiation is usually in the area right beside a weld joint in the so called melt zone. Later the crack propagates parallel to the weld joint. The tendency for reheat cracking in a material can be calculated with the equation 3 [2,4].
G = Cr% + 3.3Mo% + 8.1V % − 2 (3)
The G-value should be below zero to minimize risks for reheat cracking.
13 Alternative methods for heat stress relief
4 Process description rear axle housings
Most parts in rear axle housings are manufactured from sheet metal. The process begins at the sheet metal intake were the sheet is transported to the shearing machine. After shearing the sheet is formed to desired geometry in a forming machine. The formed part is called banjo half and is washed after forming. Washed banjo parts are later transported to the weld department. After the welding the rear axle housings are transported to the machining department from complete machining. See figure 9 for an easy overview of the production steps.
1. Tack welding & Welding
2. Machining
3. Heat stress relief
4. Surface treatment
5. Complete machining
Figure 9. Production steps
14 Alternative methods for heat stress relief
5 Method selection process After finished theory studies and process description alternative solutions for heat stress relief was investigated. This was made primary through information search from the Internet and the University library but also from personnel interviews. Scania Sao Paolo has been involved during this process and they have also contributed to some ideas that were investigated. Based from these sources three alternative methods was found and further investigated, see Table 2.
Table 2. Methods for stress relief Bend- Type: Heat stress relief Vibratory stress relief straightening Method Tested method, simple Not tested in Sweden Used by competitor Not certain, approx. 20-40min Cycle time High (multiple objects possible 140-180 sec. simultaneously) Automation High grade High potential High grade Space Big Small Average Mobility Not mobile Mobile equipment Not mobile
Purchase High Medium High
Operation 100 000SEK/year (10% High approx. 1 000 000SEK/year 100 000 SEK/year (10% of oven cost) cost of oven cost) Maintenance
Environment High Low Low effects Tests are required before Possible to change material properties Tests are required before purchase, Other purchase No need of existing Possible to log data. straightening machine? Good references e.g. NASA, US Air Possible to simulate at
force, GM mm FF Less blasting required Less blasting required No method, Hot formed Type: No method banjo Method - Used by competitor Cycle time - - Automation - - Space - - Mobility - -
Purchase - - Operation - - cost Maintenance - -
Environment - - effects Requires hot formed banjos, not an Reduces lifetime with X% Other good alternative for Ferruform
Which is equivalent to a X% reduction Could be an option for Scania Sao of fatigue strength Paulo Axle ends were not parallel after
testing (acc. to tests year 2002)
15 Alternative methods for heat stress relief 6 Temperature dependence during forming Scania Sao Paolo mentioned that a supplier of banjo halves claimed that there is no need for heat stress relief if the banjo halves are hot formed. This statement was then in the scope of further investigation to verify its authenticity.
6.1 Hot forming The definition of hot forming can be explained as the plastic deformation of a metal above its recrystallization temperature. All metals have different recrystallization temperature, because of that the method is not necessary to take place at high temperatures. An example is tin that can be hot formed at 25ºC which is its recrystallization temperature. Generally the temperature has to reach beyond 0,6 times the melting temperature.
Hot forming result in deformation of FCC austenite which is a rather weak structure compared to BCC ferrite which exists at lower temperatures.
When a material is to be hot formed it has to be heated just as much as when the press tool makes contact the final temperature of the material becomes equal to the recrystallization temperature. If the temperature is higher extensive grain growth takes place which causes strength reduction. Smaller grain size in the material leads to increased mechanical properties. The final temperature of the material is just below the recrystallization temperature. Because of that no deformation hardening is achieved which is beneficial leading to increased dimension tolerances and better surface finish.
6.1.1 Benefits • No deformation hardening results in no increase of yield strength and no decrease in ductility and hardness • Increased formability without risk of fracture • Smaller equipment and press force is needed • More homogenous material is achieved , impurities brakes down and are distributed equally over the entire surface
6.1.2 Disadvantages • Poorer tolerances as a result of thermal contractions and eventual inhomogeneous cooling • Inhomogeneous cooling can cause cracking and internal stresses • Energy demanding process • Causes scaling at high temperatures • The press tool should be heated which leads to more wear of the tool • Poorer working environment
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6.2 Cold Forming The use of cold forming is based on the strain property of the material and the strain property depends on the metal structure. There are two primary factors which are important when cold forming a metal. The magnitude of the yield strength decides how large press force needed to cause plastic deformation and how much strain allowed before rupture. Cold forming takes place at temperatures below 0,3 times the melting temperature. When a metal is cold formed it generally has to be deformed beyond the desired geometry because of the springback phenomena which always occur. How much the metal has to be deformed beyond the desired geometry is dependent on Young’s modulus which varies with different material. In order to get desired product geometry press tools have to be designed with springback in consideration. Cold forming is generally preferred for smaller components.
6.2.1 Benefits • No heating needed before forming • Higher surface finish • Superior dimension control is achieved which decrease the need of post machining operations • Flexible equipment and better form repeatability • Strength, fatigue and wear properties enhanced • Better working environment
6.2.2 Disadvantages • Higher press force needed • Heavier and more powerful equipment needed • Decreased material ductility • Metal surfaces must be free from scale • Deformation hardening occurs that might need heat treatment afterwards • Harmful direction dependent properties can arise • Unwanted residual stresses arises
6.2.3 Increase of residual stresses leads to: • Higher strength • Reduced ductility • A certain increase of density • Decrease of electric conductivity • Increase of thermal expansion coefficient [4] • Decreased corrosion resistance, particularly stress corrosion resistance
The amount of residual stresses that occur during welding is most likely to be much larger in cold formed materials compared to materials that have not been cold formed. Because of that welded constructions that have been cold formed are more likely to diverge from the desired geometry. The increased residual stresses may be dependent on the increased thermal expansion coefficient due to cold forming. This contributes to an increased expansion of the material when it is exposed to high temperatures. So when welding a cold formed material higher residual stresses are built up inside.
17 Alternative methods for heat stress relief Equation for how stress depends on the coefficient of thermal expansion. See equation 4.
σ = αE∆T (4)
σ = Stress [MPa] α = Coefficient of thermal expansion [(°C]) −1 E = Modulus of elasticity [GPa] ∆T = Temperature change [°C ]
Typical value for coefficient of thermal expansion of steel is12×10−6 °C −1 . Because of the temperature dependence the magnitude of the coefficient increases with rising temperature.
Figure 10. The coefficient of thermal expansion influence on geometry [6]
6.3 Warm forming Warm forming is defined as plastic deformation at temperatures between hot and cold forming.
Benefits compared to cold forming • Less stress on tools and equipment • Increased material ductility • Less deformation hardening reduces the need for post stress treatment • Easier to form complex geometries and possibilities to use other materials • When forming high carbon steels soft annealing is not necessary prior to forming.
Benefits compared to hot forming • Less energy consumption • Little or no scaling forms • Increased dimensional stability and surface finish • Less scrap • Less wear of tools although 25-60% higher force is demanded to form material. This is due to less thermal fatigue when tool is in contact with the heated specimen.
18 Alternative methods for heat stress relief Warm forming is still undergoing research and development and there are some obstacles still to overcome. The material behavior is less characterized at these temperatures. Lubricants have not been fully developed at these pressures and temperatures. Construction of tools has not been established specifically for warm forming.
6.4 Springback An investigation [3] was conducted concerning springback of high strength steel after hot and warm forming. A series of tests were carried out and the effect of working temperature on springback was evaluated. The result showed that springback reduces considerably when temperature rises over 750K. It is the critical temperature when recrystallization of ferrite grains begins. This experiment was verified with measurements of microstructure before and after forming and also with Finite Element Analysis.
6.5 Comparison between hot/warm forming and cold forming of rear axle housings
Report [5] shows that the material which is used in manufacturing of rear axle housings theoretically could be heated to 620°C without loss of material properties. For precaution reasons the supplier recommended that the sheet metal should not be heated higher than 550°C. At this temperature no scaling is formed, yield strength is reduced by 50% and percent elongation is increased considerably. The material can be sustained at this temperature for five hours without loss of material properties.
Hot forming at 900°C further increases the formability. Although more energy is needed and descaling is necessary afterwards. Furthermore, the material can not be used at this temperature.
6.5.1 Tension test result Tension test at 550°C and at room temperature was performed at Rautaruukki [5] and following results was presented:
• Percent elongation increased from 23% at RT to 60% at 550°C • Less force is needed to deform material, 560MPa at RT and 300MPa at 550°C • Smaller radius can be achieved, from 18mm to 6mm. (50% of sheet thickness)
Microstructure and forming velocity is two factors that are important for the material formability. If the forming velocity is low the material is able to recrystallize which makes the material more formable. But when working temperature is set to 550°C the influence of microstructural change is insignificant. This conveys that the forming velocity could be the same as earlier and would not affect the formability.
These tension tests have been conducted on a material without silica. Furthermore, some problems regarding calibration of test equipment prior to testing arose. Taking this into consideration test values should even though give a good indication of the formability of the material depending on the temperature. This report could also work as a source for further investigation.
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7 Bend-straightening
7.1 Method Bend-straightening is a method where the work piece is measured and two anvils are placed at each end, see Figure 11. The amount of bend in the work piece is detected during measuring and the bend should point towards the press direction. During straightening the work piece is bent over the yield strength of the material until desired plastic deformation is achieved. By pressing the work piece to a certain degree of plastic deformation the residual stresses are reduced. This has been confirmed empirically. There is a machine available on the market which is used by a competitor to Ferruform.
Figure 11. Bend-straightening
7.2 Summary of read reports A well known fact is that fatigue strength and life on welded constructions are affected considerably by residual stresses around the weld joint. When external forces are applied on a work piece with residual stresses unexpected deformations takes place and a higher risk of rupture is attained.
The residual stresses are not uniform but they will relax or redistribute when the construction is in use. The relaxation of the residual stresses will take place when the sum of external and residual stresses reaches beyond the yield strength of the base material. Noteworthy is when a welded construction is exposed to a cyclic load the residual stresses are considerably reduced after the first cycle and gradually decreasing until 105 cycles. Nevertheless the relaxation curve is not as steep as during the first load cycle.
7.2.1 Relaxation of residual stresses through cyclic load In the reports [3, 4] monotone and cyclic load has been used to evaluate the relaxation effect of residual stresses. The purpose has been to construct a model to predict the amount of relaxed residual stresses.
When the sum of residual stresses and the externally applied stress reached beyond the yield strength of the material, dislocation movement occurs. This is accompanied by local plastic deformations. Some of the elastic strains related to the residual stresses are converted to plastic deformations. Consequently all of the above mentioned leads to a decrease of residual stresses. The primary factors that affects relaxation is the sum of stresses and the number of load cycles. A high load imposed to the material leads to a large relaxation even after a few cycles. Higher load equals larger amount of relaxation. See Figure 12.
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Figure 12. Relaxation of longitudinal and transverse residual stresses in a material with repeated loads [3]
The specimens used for this experiment is viewed in Figure 13. Holes have been drilled in the middle of the weld joint to control the fatigue initiation sequence. The specimens are exposed to tensile stress but the effect of bend-straightening of rear axle housings is similar. The weld at the downside of rear axle housings is also exposed for tensile stress as it is bended downwards. This is why the theory found in the reports can be associated with a bend- straightening operation.
Figure 13. Specimen of material SM490A [4]
There are a number of different types of cycle loads and each one affects the amount of relaxation, repeated cycle load is most suitable for relaxation, see Figure 14 and Figure 15 on next page.
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Figure 14. Reversed load, R= σmin/σmax= -1
Figure 15. Repeated load, R= σmin/σmax= 0
A model has been developed for predicting relaxation of welding residual stresses, see equation 5. [4]
[()σ + σ ] res ini app ≥ 1 (5) σ y
()σ res ini = Initial residual stresses σ app = Applied stress σ y = Yield strength of base material
Relaxation can not take place unless the ratio of the above equation exceeds 1. If not, the relaxation of residual stresses will not initiate. When the ratio reaches 1.6 full relaxation is achieved, above this ratio compressive stresses are built into the material.
It has been found that residual stresses decreases considerably after one load cycle. The relaxation continues with increased number of cycles, but not in the same extent.
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7.3 Result
”Residual stress relaxation by reversed loading”
• With reversed loading (R=-1) the residual stresses decrease considerably after one cycle and a gradually decreases with increased load cycles. The longitudinal residual stresses are reduced with 39 % after one cycle with an applied load of 78 % of the base material yield strength. After 104 cycles with the same applied load, residual stresses are decreased additionally to 54 %.
”Relaxation of welding residual stresses by reversed and repeated loadings”
• Both reversed and repeated cyclic load reduces residual stresses considerably after one load cycle. Higher load equals larger amount of relaxation
• The amount of relaxed residual stresses by repeated loading is larger than relaxation by reversed loading
• At repeated load cycles and an applied load of 300 MPa (72 % of yield strength) the longitudinal residual stresses are reduced from 227 MPa to -69 MPa thus compressive stresses are introduced. The corresponding reduction in transversal stresses is from 57 MPa to -7 MPa thus an introduction of compressive stresses. These relaxations occurred after one cycle load.
• At reversed cycle loads and an applied load of 200 MPa (78 % of yield strength) the longitudinal residual stresses are reduced with 39 % after one cycle load. Corresponding reduction in transverse stresses are 59 %
• In the base material a certain amount of compressive stress relaxation occurs after one cycle load. At repeated cycle loads from -109 to -37 MPa thus a reduction with 34 %. The applied load in this case was 300 MPa (72 % of yield strength)
”Residual stress relaxation of welded steel components under cyclic load”
• Residual stresses are relaxed if the sum of the applied stress and the residual stress exceeds the yield strength of the material. The degree of excess determines the amount of relaxed stresses. • The residual stresses are reduced considerably after one cycle load. Following number of cycle loads decreases stresses further but not in the same extent as after one cycle.
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7.4 Example of a bend-straightening machine Because of sensitive information pictures can not be published.
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7.4.1 Automatic straightening of rear axle housings The bend-straightening procedure can not be published because of sensitive information.
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8 Stress relief by vibration2
8.1 History As early as in 1943 during Second World War advantages with vibrations was discovered. Welded constructions which were transported from the weld shop to the machine shop by railway or by truck showed less distortion when machined. As the war progressed the Germans introduced “transport after welding” on large precision components. Plant managers and engineers did not know why this phenomenon occurred, but they thought it was a result of movements in the work piece. They started to experiment and developed methods to generate movements in a more controlled manner.
At the same time in the USA the number of manufacturing industries increased in the war sector, both in number and size. The demands also increased on precision metal components. The Americans also discovered that through induced vibrations in the material the constructions gained in precision.
8.2 Background Welding is an efficient and reliable process that is widely used within the industries for attaching metals. But during welding residual stresses are induced in the work piece due to extensive heating and quick cooling. The residual stresses affect the construction negatively, because of a reduction in fatigue, increased distortions and higher risk of welding cracks. It is important to control and decrease the residual stresses caused by welding and there is a number of methods that can be used to accomplish this. Heat stress relieving is a widely used method that works very well but requires a large oven and is time and energy consuming. Another method that is available is vibratory stress relief of constructions and it is described below.
8.3 General info A work piece with residual stresses caused by heating can be compared to an instrument out of tune. It means that during a control of the harmonic frequency it will be out of its natural position. The harmonic frequency of the work piece has changed due to residual stresses; this change can be used to verify stress relief.
During heat treatment of a work piece the heat reduces the yield strength so that the residual stresses exceed the yield strength. This brings local plastic deformation which reduces the intensity of the residual stresses. Also a reduction in hardness is a result of the heating.
The atomic structure in the work piece that is affected by residual stresses feels no difference in energy induced by heat or energy induced by vibrations. The supplied energy rearranges the crystal structure, i.e. the atoms changes position. Thus residual stresses are reduced without any distortions.
2 References to this chapter can be found at [1,2,3,4,5,6]
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8.4 Implementation of vibratory stress relief Vibratory stress relief is a relatively simple method that induces harmonic or sub-harmonic vibrations with high amplitude in the material. Regular equipment for vibratory stress relief consists of an electric source with a control unit, an electric motor that rotates eccentric weights (force inducer) and an accelerometer (transducer) that registers the frequency flow through the work piece. See Figure 17 and Figure 18 for an overview of typical arrangements. The vibrations are induced in the work piece in two ways. If the work piece is big enough the vibrator can be attached directly on it. The work piece has to be isolated from the floor with rubber pads, these prevent the vibrations from spreading to the surroundings. Another way is to attach the vibrator to a table where the work piece is attached, see Figure 19. The table is also isolated by rubber pads which makes stress relief possible for many work pieces simultaneously. Small parts can not be stress relieved without the table because the harmonic frequency is out of range for the vibrator. The frequency of the vibrations depends on the material, size and shape of the work piece. Usually the vibration frequency is in the range of 0-100Hz. The time for vibration treatment is often less than 30 minutes.
Currently there are two types of vibratory stress relief methods available. During the first method the work piece is vibrated and scanned slowly from zero frequency to maximum frequency, e.g. 0-100Hz for 8 minutes. The response is analyzed and its harmonic frequency is registered. Usually two or three of those frequencies are registered. The vibrator is adjusted and changes speed to one that is equal to the first harmonic frequency. The vibration continues a given time, usually 10 minutes. After this the speed of the vibrator is increased to one that is equal to the next upper harmonic frequency and the process repeats. The other vibratory stress relief method, called sub-resonant has an initiation procedure that is the same where the harmonic frequencies are registered. The frequency for the vibrations is then held slightly below the harmonic, usually 4Hz below. When the first harmonic frequency is decreased and stabilized the work piece has been relaxed from residual stresses. The time for this to happen is approximately 20-30 minutes depending on the size of the work piece. See Figure 16 for a description of the sub-resonant method.
8.5 The mechanism behind vibratory stress relief When forced vibrations with variable frequencies affects a work piece both the harmonic frequency and the induced frequency will act on it. When the induced frequencies coincide with the harmonic frequency, these energies become additive. This results in an increase of vibration magnitude and the frequency at this point is called the resonance frequency.
All metals have a harmonic frequency. The metal will vibrate at this frequency when it is exposed to a sudden impact for e.g. when hammering on a beam. After the blow the metal vibrates until that all of the energy is dissipated through internal friction.
Vibratory stress relief affects the material similar as with the hammer blow. The difference lies within the source of vibration; vibratory stress relief uses an electrical motor that causes constant supply of vibrations.
When a work piece is vibrated heat builds up and causes an infinitesimal increase of temperature inside the material. Energy from vibrations has the same effect as heat stress relieving when looking at the general stress relief of the lattice structure. The supplied
27 Alternative methods for heat stress relief vibrations induce enough energy for the dislocations to reposition and return to a level of less stress. In this way the residual stresses are reduced in the material.
8.6 Vibration during welding There is also a vibration method that is used during welding (vibratory welding conditioning, VWC). With the vibrator attached and activated on the work piece during welding better distortion control is achieved. Also welding cracks are minimized and fatigue life is increased. This is due to that welding additive is better drawn into the joint and cooling rate is more uniform. Because of this less residual stress is built up and less current is needed during welding. In a metallurgical point of view the vibrations during welding gives a more uniform grain size.
8.7 Reflections Currently there are a number of suppliers of this equipment which uses two different methods, stress relief by harmonic frequencies and sub-harmonic frequencies. It is difficult to determine the best method just reading about them because both methods are advocated by the suppliers, and they both states that their method is the most advantageous. Scientific research has been conducted but the results are not consistent. The effect and time needed for vibratory stress relief depends on size, shape, strength, elastic modulus and weight of the work piece. These factors decide the ability for residual stresses to relocate and balance up in the work piece.
The only way to really find out the effect of the different methods is to test it on the specific product and compare it to the current method for stress relief.
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8.8 Diagram and pictures
Figure 16. Operation procedure for sub-resonant Vibratory stress relief
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Figure 17. Vibratory stress relief of multiple work pieces
Figure 18. Vibratory stress relief of one work piece
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Figure 19. Vibratory stress relief of multiple pieces attached to a table
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9 Tests with alternative methods for heat stress relief
9.1 Vibratory stress relief test
9.1.1 Description In order to decrease residual stresses caused by extensive welding operations vibration stress relief can be used. This test was made with equipment from Meta-Lax which is a supplier of a sub-resonant vibratory stress relief technology. Six rear axle housings were vibrated one at a time.
9.1.2 Method Before the test a fixture had to be made were the rear axle housing could be mounted. The fixture was first constructed in a CAD program called Unigraphics NX5. See Figure 20 below.
Figure 20. CAD image of vibration fixture
The rear axle housing is firmly attached between two v-blocks to ensure metal-metal contact. This is done by nuts and bolts. Metal contact is important to get good distribution of the vibrations. Also the force inducer has to be firmly attached to the fixture and this as done by clamps. A control unit is used for tuning in the force inducer e.g. finding the resonant frequency. See Figure 21 and Figure 22 for an overview of the arrangement.
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1.
2.
Figure 21. Picture of the arrangement; 1. Force inducer, 2. Transducer
Figure 22. Control unit
33 Alternative methods for heat stress relief The stress relief method is performed with a number of steps, see below.
1. First scan o The operator gradually increases the frequency of the force inducer from the control unit until the resonant frequency for the rear axle housing is found.
2. Dwell time o The frequency of the vibration is held at around 4Hz below the resonant frequency and maintained for a certain time depending on the object, in our case about 20 minutes.
3. Second scan o The resonant frequency is located again and then compared to the first scan. If the resonant frequency is altered from the first scan stress relief has taken place.
4. Verification dwell time o To ensure that the stress relief is finished a second dwell time of about 5 minutes with a frequency of 4 Hz below the new resonant frequency is needed.
5. Third scan o If the third scan coincide with the second scan one can know that the stress relief is finished.
Figure 23. Example of a run with vibratory stress relief 3
3 Picture can be found at www.bonal.com
34 Alternative methods for heat stress relief 9.2 Bend-straightening test
9.2.1 Description In order to decrease residual stresses caused by extensive welding operations bend- straightening can be used. The test was made in order to simulate a bend-straightening machine that is available on the market today.
9.2.2 Method Six rear axle housings were bent in four directions, two bends in the horizontal plane and two bends in the vertical plane. This was made with a 40 tonne hydraulic press and a fixture constructed in a CAD program called Unigraphics NX5, see figure 24.
Figure 24. CAD-picture of the bend straightening fixture
The ends of the rear axle housing were supported by fixtures in order to bend the housing at its centre. The idea was to get a bend of 10mm in its centre in all four directions. A measuring device was placed at the top center of the rear axle housing to detect the amount of bending, see table 2 on next page for test results.
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Table 3. Results of the bend straightening test Bending [mm] Remaining deformation [mm] Nr Direction 1 Direction 2 Direction 3 Direction 4 Direction 1 Direction 2 Direction 3 Direction 4 1 10 10 5 4 1 1,9 0 0,18 2 10 10 4 4 1,22 1,5 0,18 0,68 3 10 10 4 4 1 1,7 0,08 0,6 4 10 10 4 4 1,27 1,55 0,14 0,21 5 10 10 4 4 1 1,5 0,32 0,2 6 10 10 4 4 1,5 1,55 0,55 0,33 Average 10 10 4,17 4 1,17 1,62 0,21 0,37
When remaining deformation was detected this was taken into account when bending in the opposite direction. For example nr 2 had a remaining deformation of 1,22mm and then the opposite bend was 10 plus 1,22 mm equals 11,22mm.
The pictures below show the different bending directions.
Figure 26. Direction 2 Figure 25. Direction 1
Figure 27. Direction 3 Figure 28. Direction 4
36 Alternative methods for heat stress relief 9.3 Conclusions
9.3.1 Vibratory stress relief The test showed that a total dwell time of about 20-25minutes was needed for the vibratory stress relief. Before the test there were some questions regarding noise levels and the vibration effect on the surroundings. We could establish that the noise level was very low in general but with some peaks in the scans when locating the resonant frequency. The important thing was that during dwell time noise level was not a problem. The effects on the surroundings were not a problem; the vibrations were stopped by the rubber pads beneath the fixture.
9.3.2 Bend straightening During the first attempt there was a problem with the hydraulic cylinder, it did not produce enough force to bend the rear axle housings 10 millimeters in the vertical plane. The maximum bend possible was about 5 millimeters e.g. half of the planned bending. Therefore it was decided that the vertical bend limit was set to 4 millimeters instead of 10. This was due to no optional equipment, short time limit and also a high risk for deformation of the press area. Deformation occurred on one rear axle housing in the press area when pressing just 5 millimeters in the vertical plane. Although this problem may have been due to a small contact area between the hydraulic cylinder and the vertical plane. After deformation occurred the stamp was changed to one with a larger area and the vertical plane was bent 4mm, then no deformation occurred at the contact area. The horizontal bending of 10millimeters was not a problem at all.
9.3.3 Verification To ensure the effect of these methods a verification test is needed. This can be done with a number of different tests such as fatigue strength test or residual stress analysis. The idea of the verification test is to compare the heat stress relieved rear axle housings with the rear axle housings that are stress relieved with vibrations and bend-straightening.
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10 Test with blasted banjo halves
10.1 Background On account of a possible introduction of a new method for relaxation of rear axle housings the effects of disposing heat stress relief has to be investigated. It is a well-known fact that heat stress relief is soiling the rear axle housings with soot and a certain amount of scaling take place. A new method will not give these problems which of course is beneficial.
The purpose with this test was to investigate how rear axle housings would look like post welding if the rear axle housing halves were blasted before any welding operation. Maybe additional blasting becomes unnecessary or a reduced cycle time could be obtained.
10.2 Performance
1. Banjo weld of blasted rear axle housing halves o The blasted halves are welded together in the Banjo weld station
2. Welding of cover and ring o Cover and ring are welded on the Banjo part
3. Finish weld o Finish welding of rear axle housing
4. Blasting o Two rear axle housings were blasted with today’s cycle time and two were blasted half the time without internal lances.
Between each step pictures were taken to document possible differences during the course of events. See the following pictures.
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10.3 Result
10.3.1 After Banjo weld
Figure 29. Blasted rear axle halves after Banjo weld
Oil residue is clearly showing in this early stage. This oil residue probably has its origin from the Valmet mill at the joint preparation station. An indication of possible weld improvement could be seen because the weld pool had floated out more.
10.3.2 After welding of cover and ring
Figure 30. Rear axle housings after welding of cover and ring
Additional oil residue is showing. This residue probably comes from contour and end milling or plane milling which is done before welding.
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10.3.3 After finish weld
Figure 31. Finish welded rear axle housing
Moderate levels of residue caused by welding are showing around spring brackets and reinforcement plates. The weld joints around these do not look poorer than in normal production. Yet blasting of the rear axle housings will still be necessary to detect defects at the following repair weld station. Blasting is also necessary to obtain a clean surface before primer painting. No remaining oil residue could be seen.
10.3.4 After blasting the half time without lances
Figure 32. Side view of rear axle housing Figure 33. Inside of rear axle housing
Figure 34. Label glue residue
40 Alternative methods for heat stress relief 10.4 Conclusions It can be established that blasting half the time without internal lances gives a good result. No oil residue or weld residue remains. The inside has a satisfying result despite of no internal blasting was done. However when looking at Figure 34 it can be seen that some of the label glue remains and this can cause adhesion problem for the primer paint. Noteworthy is that two of the rear axle housings that were blasted full time also had some label glue left, yet slightly less.
10.5 Discussion High occurrence of oil residue could be seen in the beginning of the production. The oil residue had vanished after blasting which is good. Though invisible oil encapsulement in the surface may occur and this can cause adhesion problem for the primer paint. There is also a higher risk for blasting media to lump up in the blasting machine.
It is generally known that welding of blasted metal gives less weld deviation and thus an increase of welding speed is possible.
When introducing an alternative method for heat stress relief there is a potential for reducing the cycle time for the blasting operation after finish weld. Although consideration has to be taken to the fact that the halves were blasted before the welding operations and this is not done today. The blasted halves contributed to the clean inside of the rear axle housings.
Important to point out are the label glue remains after blasting. These are eliminated today in the heat stress relief oven due to the extensive heat. Therefore a new method for marking the rear axle housings has to be considered if the heat stress relief oven is exchanged with a new method for relaxation.
Before the test was performed there was a discussion whether blasting the halves would increase the risk for oxidation of the surface. This because of that the protecting scale is removed after blasting. It was established that there was not a problem with this. The surface seemed to be unaffected during the test despite that is lasted from v36 to v41, thus 5 weeks. But this is season dependent with different air humidity in the plant. Also the placement of the rear axle housings when stored can affect the amount of oxidation.
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11 Influence of production change
11.1 Background One important part in this project was to investigate the influence of manufacturing process due to an introduction of a new method for stress relieving rear axle housings. When introducing a new method consideration has to be taken to a number of important factors that will affect the current situation.
A summary of the different method suggested are listed below. Also the positive and negative aspects are taken into account and explained.
11.2 Bend-straightening
11.2.1 Summary If the oven used for heat stress relief is exchanged with a bend-straightener the production process is going to be significantly altered. The main thing when introducing this equipment is that the production flow will not be controlled in batches by six as today. Also the space required for the equipment is reduced significantly, see Figure 36. There is currently one competitor manufacturing rear axle housings that uses a bend-straightening machine. The machine has been proven successful and at least 3 machines have been delivered to their plant. Although consideration has to be made regarding implementing this machine at Ferruform. The competitor is using hot formed banjo parts for their rear axle housings, Ferruform are using cold formed banjo parts.
11.2.2 Main factors
1. Possible dispose of current straightening operation 2. Layout at the workspace 3. Production control 4. Blasting 5. Primer painting
1. Possible dispose of current straightening operation Currently the rear axle housings are straightened after cover and ring welding operation. This is due to improve dimension stability during finish welding and machining operations.
+ Time saving o When introducing a bend-straightener for stress relief of rear axle housings it is possible to do both relief and straightening in the same machine. This leads to that the current straightening machine becomes an excess and can be disposed. The time savings due to dispose will result in about one minute per rear axle housing
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+ Resource saving o The current straightening operation requires an operator in order to secure the production flow. The machine is controlled manually. A dispose of the current straightening machine would result in a reduction of manpower.
- Joint following o It is uncertain if the weld gun can manage to follow the joint during finish welding of the rear axle housings if the current straightening machine is disposed. Although finish welding without prior straightening have not been tested and may very well work. A test is needed to clear this thing out.
2. Layout at the workspace
+ Less space required o Space required for the bend-straightening machine is significantly less than the oven used today. Approximately 50% of the current space is needed. For more detailed information see layout at Figure 36.
- Robotic handling o In order to manage rear axle housings in the production line an automated robot cell is required for loading and unloading the bend-straightening machine. This is negative due to costly equipment
3. Production control Heat stress relief oven uses batches of rear axle housings to meet a cycle time of X minutes. The heat stress relief oven is divided into 8 sections, pre-heat, dwell and cooling. One section is always occupied by one batch and the cycle time for one batch to travel from inlet to outlet is X hours. This means that every X minute one batch exits the oven and when every batch contains X rear axle housings the pace is X minutes.
+ Batching o The bend-straightening machine handles one rear axle housing at a time. Because of the low cycle time of this machine no batching is needed to meet the required pace time.
+ Flow o Because of the single handling of rear axle housings the flow is evenly distributed through the production line. This makes handling easier and also improves internal logistics.
43 Alternative methods for heat stress relief 4. Blasting When heat stress relief is used scaling and soot occurs on the rear axle housings. This is removed by the blasting operation. Blasting is also necessary to remove weld residue caused by prior welding operations. A third important factor is to bring fourth eventual weld defects that are hard to detect prior to blasting. Theses eventual defects are then detected during repair welding after the blasting operation. Yet a positive effect of heat stress relief is that potential oil residue from earlier operations is eliminated by the heating process.
+ Possible cycle time reduction o As a result of removing the oven no scaling or soot occurs. This means that the blasting operation cycle time could be reduced.
- Oil encapsulation o When introducing bend-straightening no oil removal effect is attained and potential oil residue remains on the surface of the rear axle housings. During blasting this potential oil residue can be forced into the material and cause trouble in the following primer paint process.
- Polluted blast medium o When oil residue is left on the rear axle housings during blasting the blasting medium absorbs some of the oil. This can cause the medium to lump together and eventually the blasting machine will not work properly
5. Primer painting
- Oil encapsulation reduces surface adhesion and causes primer paint to flake off.
44 Alternative methods for heat stress relief
11.3 Vibratory stress relief
11.3.1 Summary If the oven used for heat stress relief is exchanged with vibratory stress relief some alteration in the production process is to be expected. First of all the oven is replaced by 4 stations with vibratory equipment and each station can manage X rear axle housings simultaneously, see Figure 37. This configuration is based on calculations of a cycle time of X minutes as the main factor. Another important thing is that the vibratory method does not alter the mechanical properties of the material.
11.3.2 Main factors
1. Layout at the workspace 2. Production control 3. Blasting 4. Primer painting 5. Environmental effect
1. Layout at the workspace
- Robotic handling o In order to manage rear axle housings in the production line an automated robot on railway is required for loading and unloading the vibration stations. This is negative due to costly equipment
± Uncertain if less space is required o Due to the fact that X vibration stations is needed to manage the pace it is uncertain if the space required is any less than the current space used. Although there is no problem to fit the stations within the space available.
2. Production control
± Batch size o X rear axle housings are put into each station so the batch size is reduced from X to X.
3. Blasting When heat stress relief is used scaling and soot occurs on the rear axle housings. This is removed by the blasting operation. Blasting is also necessary to remove weld residue caused by prior welding operations. A third important factor is to bring fourth eventual weld defects that are hard to detect prior to blasting. Theses eventual defects are then detected during repair welding after the blasting operation. Yet a positive effect of heat stress relief is that potential oil residue from earlier operations is eliminated by the heating process.
+ Possible cycle time reduction o As a result of removing the oven no scaling or soot occurs. This means that the blasting operation cycle time could be reduced.
45 Alternative methods for heat stress relief
- Oil encapsulation o When introducing vibration no or minimal oil removal effect is attained and potential oil residue remains on the surface of the rear axle housings. During blasting this potential oil residue can be forced into the material and cause trouble in the following primer paint process.
- Polluted blast medium o When oil residue is left on the rear axle housings during blasting the blasting medium absorbs some of the oil. This can cause the medium to lump together and eventually the blasting machine will not work properly
4. Primer painting
- Oil encapsulation reduces surface adhesion and causes primer paint to flake off.
5. Environmental effect
- Low frequent vibrations may have a negative effect on surrounding environment. Operators that are working nearby for a long period of time could take damage from these vibrations.
46 Alternative methods for heat stress relief
11.4 Hot formed banjo parts
11.4.1 Summary Hot formed banjo parts are an alternative forming method compared to cold forming. As can be seen in chapter 6 residual stresses are reduced and heat stress relief may be unnecessary. Scania Sao Paolo is in an expansive stage were they are considering options for their heat stress relief oven. Except for the methods described above hot formed banjo parts is also an alternative. This is because they are able to buy pre-fabricated hot formed banjo parts instead of the cold formed bought today. This is not an option for Ferruform because of the extensive reconstruction of current equipment which leads to high costs, see below for pros and cons.
Ferruform
- Bad work environment - Requires new equipment or extensive reconstruction - Higher operation cost - More expensive tools - High demands on fire safety regulations - Blasting have to be done because of scaling occurs
Scania Sao Paulo
+ Possible to buy hot formed banjo parts from sub-supplier + Heat stress relief unnecessary + Bend-straightening a potential, same procedure as competitor
Before a new method is implemented the result has to be tested and compared with the result of the method currently used. The test method commonly used is fatigue tests.
47 Alternative methods for heat stress relief
Figure 35. Layout of available space
48 Alternative methods for heat stress relief
Figure 36. Layout Bend-straightening
49 Alternative methods for heat stress relief
Figure 37. Layout vibratory stress relief
50 Alternative methods for heat stress relief 12 Conclusion It is established from prior work that stress relief of rear axle housings is necessary for fatigue strength dimension stability. However, rear axle housings without stress relief can be machined after welding without diverging from the specified tolerances.
The tests that have been made prove that current blasting cycle time can be reduced by implementing a non heat stress relief method. This is due to that no scaling occurs from heating the rear axle housings.
Tests made with bend-straightening and vibratory stress relief have to be verified in order to evaluate the stress relief effects from the different methods. Hot formed banjo halves are also an interesting alternative seen from Scania Sao Paolo’s point of view.
These three methods have been theoretically analyzed and show a good potential for stress relief of welded components.
From the different methods bend-straightening would be the best solution from Ferruform’s point of view. This is based on the fact that bend-straightening handles one piece at a time with short cycle times and is also used by a competitive manufacturer. There is also a possibility for disposal of the current straightening operation that is performed today. This operation can be combined with the stress relief straightening operation.
Noteworthy is that the test with bend-straightening was only a simulation of the real operation
Important to point out is that the vibratory stress relief method has a potential of stress relieving components during welding. This decreases the need for stress relief after welding.
Energy consumption and space are reduced regardless of which alternative method chosen.
Alternative methods for heat stress relief
13 Discussion and recommendation A verification test has to be performed on the stress relieved rear axle housings. A fatigue strength test with random loads is a method which gives reliable results. This will be a basis for evaluating the effects from the alternative methods.
Consideration has to be taken to the verification result of the simulated bend straightening test. The result does not have to be 100% correct because of the fact that it was only a simulated test. The real bend-straightening machine performs the stress relief in a more controlled manner but the verification result will still give an estimation of the potential stress relief.
Alternative methods for heat stress relief
14 References
References chapter 2
1. Klas Weman (2002), Karlebo Svetshandbok 2. P Ahlberg, J Hedegård (1988), Avspänningsglödgning av svetsade konstruktioner 3. www.svets.se 4. Arne Anderdahl, Per Sjölin (1977), Behov av avspänningsglödgning efter svetsning av varmhållfast stål typ 1Cr0.5No 5. William D. Callister, Jr (2003), Materials Science and engineering an introduction, sixth edition 6. www.hghouston.com/x/17.html 7. www.bodycote.se
References chapter 5 1. E.Paul DeGarmo, J Temple Black, Ronald A. Kohser (1988), Materials and Processes in Manufacturing, Seventh edition 2. William D. Callister, Jr (2003) Material science and engineering an introduction, Sixth edition 3. Cirp Annals 2005 Manufacturing Technology vol. 54/1 p.213, Springback of high- strength steel after hot and warm sheet formings 4. http://www.mechanicalengineering.cc/mechanical-engineering-archives/205-Welding- of-aluminium.html, Cold forming or strain hardening 5. Project report at Luleå University of Technology (2003), Jämförelse mellan varm- och kallformning av bakaxelbryggor 6. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thexp.html
References chapter 6 1. www.mae-goetzen.de 2. www.roymech.co.uk/.../Fatigue/Stress_levels.html 3. K.Lida, S.Yamamoto and M.Takanashi (Japan), Residual stress relaxation by reversed loading 4. Seungho Han, Takkee Lee and Byungchun Shin, Residual stress relaxation of welded components under cyclic load 5. Kunihiro Iida, Masahiro Takanashi ,Relaxation of welding residual stresses by reversed and repeated loading
References chapter 7 1. S. Shankar (1982), Vibratory stress relief of mild steel weldments
2. Thomas E. Hebel (2001), Sub-harmonic Stress Relief Imroves Mold Quality
3. Giovanni S. Crisi, Danila Pedrogan Mendonca (2006), Stress relief of welds by heat treatment and vibration: A comparison between the two methods
4. Jijin Xu, Ligong Chen, Chunzhen Ni (2006), Effect of vibratory weld conditioning on the residual stresses and distortion in multipass girth-butt welded pipes
Alternative methods for heat stress relief
5. R.D Skinner, Structures Manufacturing Engineer, Lockheed Missiles and Aerospace (1987), An investigation into the theory behind sub-resonance stress relieve
6. Suppliers of vibratory stress relief equipment www.bonal.com www.airmatic.com www.stressreliefengr.com www.rrstress.com
Appendix 1: Travel reports Page 1 of 12
15 Appendices Appendix 1: Summary of travel reports These reports can not be published because they contain sensitive information.
Appendix 1: Travel reports Page 2 of 12 Manufacturing of rear axle housings at Ferruform
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Appendix 1: Travel reports Page 4 of 12 Travel report Maxion/Scania Sao Paulo Manufacturing of banjo halves at Maxion. Manufacturing of rear axle housings at Scania Sao Paulo in Brazil.
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Appendix 2: Prior investigations Page7 of 12 This page is blanked because it contains sensitive information.
Appendix 2: Prior investigations Page8 of 12 Travel report Dana Visit at the axle factory Dana/Eaton in Pamplona, Spain
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Appendix 2: Prior investigations Page9 of 12 Travel report Arvin Meritor Lindesberg Visit at the axle factory Arvin Meritor in Lindesberg, Sweden
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Appendix 2: Prior investigations Page11 of 12 Travel report EGE Endustri Turkey Visit at the axle factory Arvin Meritor in Lindesberg, Sweden
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Appendix 2: Prior investigations Page12 of 12 Travel report Arvin Meritor Osasco Visit at the axle factory Arvin Meritor Osasco, Sao Paulo Brazil
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Appendix 2: Prior investigations Page1 of 6 Appendix 2: Prior investigation heat stress relief This is a summary of the report “Fatigue strength compare of rear axle housings with and without heat stress relief””.
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Appendix 3: Specification of requirements Page 1 of 3 Appendix 3: Specification of requirements on rear axle housings
General Today Ferruform is using the method heat stress relief after finish welding of the rear axle housings in order to reduce the residual stresses built in during extensive welding. An electrical heated oven is used for this purpose and loading and unloading are done automatically except for the manual loading on the inlet conveyer. The rear axle housings are then picked up by a portable robot which is putting the heat stress relieved housings on the inlet conveyor of the blast machine.
Purpose Heat stress relief is a very time consuming operation and that is why an alternative method for reducing the residual stresses would be an advantage. Also in an energy consuming point of view an alternative method would be desirable.
Goal By implementing a new method reducing the cycle time and costs for relaxation of the rear axle housings.
Extent All rear axle housings that are heat stress relieved today shall be stress relieved with the new alternative method. The equipment has to give a similar or better result and automatize have to be possible to a high extent.
Operation description The equipment shall together with a robot handle the rear axle housings from inlet- to outlet conveyor. It must be able to manage a cycle time of X minutes for the relaxation. The result with the new method has to be similar or better then the heat stress relief.
Transport and loading Transport and loading shall be done manual to the inlet conveyor.
Existing data on rear axle housings
• Length 1600mm • Maximum rotation diameter 800 mm • Sheet steel thickness 12 mm • Weight ca 140kg
See Figure 38 and Figure 39 for an overview of rear axle housing.
Technical specification It must be specified by the supplier in the concept proposal if there are special needs concerning the building.
Appendix 3: Specifikation of requirements Page 2 of 3 Dimension description The supplier gives a description on the space demand for the equipment. The interesting dimensions are the physical size of the equipment and its working range. CAD-drawings are gladly accepted.
Working environment The requirements by law have to be fulfilled and machines must be CE-marked. Also Scania TFP04 and the local addition, DL2005 have to be fulfilled.
Environment Law requirements must be fulfilled and the equipment has to be CE-marked. Local regulations (municipality and county administration board), Scania TFP04 and the addition DL2005.
Quality Capability, see TFP04 K4.2.
Ergonomics Possible assist equipment have to be available.
Maintenance The supplier states the interval for maintenance of the construction. It has to be mentioned if the maintenance can not be performed by the personnel at Ferruform. The supplier has to provide special tools if there is a need for them to perform maintenance. Finally estimation on time and money needed for annual maintenance shall be submitted.
Availability Specified by supplier.
Education of personnel The supplier specifies the requirements needed for operating the equipment.
Technical requirements for production equipment (TFP04) TFP04 and the local addition, DL2005 shall be seen as a requirement for the equipment. TFP04 is referring to the personnel safety department. Possible departures are done in consultation with Ferruform AB.
Concept proposal Information about conceivable equipment for the purpose has to be included. This can be information regarding education of staff, budget quotation and delivery time.
Contacts Jonas Holmgren, Manufacturing engineering division, Ferruform AB Stefan Lindqvist, Manufacturing engineering division, Ferruform AB
Appendix 3: Specifikation of requirements Page 3 of 3
Overview of rear axle housings
Figure 38. Finish welded rear axle housing ready for heat stress relief.
Figure 39. Finished rear axle housing
Appendix 4: Geometry and lifetime requirements Page 1 of 1
Appendix 4: Geometry and lifetime requirements
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