Heat Treating: Proceedings of the 18th Conference Copyright © 1999 ASM International® Ronald A. Wallis, Harry W. Walton, editors, p 5-11 All rights reserved. DOI: 10.1361/cp1998ht005 www.asminternational.org

Distortion in Case Carburized Components­ The makers View

M. Cristinacce British Steel Engineering Rotherham, United Kingdom

1. Abstract NVH (noise, vibration and harshness) is becoming of increasing importance in the automotive industry. The control Distortion of components during heat treatment has a of distortion to give correct mating of rotating components significant effect upon final component costs. The factors such as gears and shafts has a major influence upon NVH which influence distortion behaviour occur during the performance. machining and heat treatment processes and are therefore Most of the examples given in this paper are automotive outside the control of the steelmaker. One important factor transmission components which are carburised. which is under the control of the steelmaker is hardenability ­ A wide variety of factors influence distortion behaviour Consistent hardenability performance can have a significant and can be broadly summarised as follows: effect in reducing the variability in distortion. In a number of Component shape. instances it has been shown that the macrostructure and Steel type. as-cast shape of the steel can also influence distortion. Other Microstructure and residual stresses prior to heat downstream processing effects such as forging may also be treatment. influential in these circumstances. Reheating and carburising conditions. This paper gives examples of some of the experiences of Stacking and support in furnace. British Steel Engineering Steels with customers and end users, - Medium, temperature, flow, jigging, etc. and refers to relevant published work. Hardenability . As-cast shape. 2. Introduction Macrostructure Of the above factors, the steelmaker has influence over The automotive industry requires that engineering steels only the last three, hardenability, as-cast shape and are made to greater levels of consistency to meet improved macrostructure. consistency of properties and processing response. This is to The remainder of the paper is therefore concerned with ensure that cost and performance targets are met in the final these subjects. product. 3. Hardenability A prime example of this is in the control of distortion during heat treatment. The hardenability of steel depends upon composition and As a result of non-uniform heat transfer and volume austenitic grain size. This relationship has been understood changes on transformation, a change in shape or distortion for some time. More recently the relationship has been will always occur when ferrous components are heat treated. developed to enable the accurate prediction of hardenability Ifthis change in shape can be controlled and made consistent based upon composition (1). British Steel Engineering Steels corrections can be made in the machining processes so that the employs this understanding during secondary components distort into "true" on heat treatment. Where this operations to achieve a very close control of composition and is not possible, product costs are increased as a result of hence hardenability (2). Cast to cast variation in hardenability losses, rectification and extra process steps such as hard is minimised which is vital to the steel user as it has a major machining after heat treatment. Controlled and consistent influence upon the component properties and the distortion distortion from cast to cast and batch to batch therefore which occurs during heat treatment. The relationship with enables component costs to be reduced. distortion has been clearly demonstrated (3).

5 Practical examples of how hardenability influences The gear is made as a ring-shaped forging which is machined distortion have been reported in published work (4, 5) and one: and carburised. example for gears in 27MC5 grade as shown Measurements of out of roundness (OOR) or ovality after below, was given by Rezel (6). heat treatment are plotted against the hardness value at the

% Rejections for Distortion 1.2

1

0.8

0.6

0.4

0.2 co 0 38 40 42 44 46 48 Hardness tiRe at J11mm

Figure 1 Influence of Hardenability upon Rejections for Distortion

Figure 1 shows % rejections for distortion against the hardness J6.5(mm) position on a Jominy hardenability test in Figure 2. value at the 111 (mm) Jominy position. It can be seen that as It can be seen that as the hardness exceeds 41HRc there is an the 111 value exceeds 42HRC, the rejections for distortion increase in OOR distortion. A minimum hardness of 38HRC is increase significantly. In order to meet the required properties required at the J6.5 Jominy position in order to meet the and avoid excessive losses for distortion, a restricted design properties of the gear and therefore a hardness range of hardenability of 37/43 or 38/42HRC at 111 is specified for 38/41HRc is required. This compares to a typical 20MnCrS5 measured and calculated Jominy results respectively. restricted range of 6HRC at this position and can only be Recently, British Steel Engineering Steels was asked to achieved by precise control of composition during produce a 20MnCrS5 type carburising steel to a very restricted steelmaking. hardenability range to control distortion in a car final drive gear.

O.O.R Distortion 1/100mm 16,------.------,

14

12

10 8 -

6 4 -

2 '----~_ __.l..__~_...i.___~_ __.J.__~_ _"__~_--' 38 39 40 41 42 43 HRC at J6.5mm

Figure 2 Influence of Hardenability upon Distortion in a 20MnCrS5 Final Drive Gear

6 The above examples show that by working to controlled hardenability limits, the steelmaker can assist in the process of distortion control.

4. As-Cast Shape

The effect of the as-cast shape upon distortion in car crownwheels in 16MnCr5 grade was reported by Seger (7), In this case, the use of a non-square continuously cast slab section in place of the normal continuously cast square section gave rise to an excessively oval shape in the crownwheels after heat treatment This was further investigated by Gunnarson (8) who demonstrated that the use of a continuously cast round section in place of a section rolled from non-square continuously cast bloom gave a reduction in ovality on large truck crownwheels, Figures 3a and 3b show schematically the differences in out of roundness distortion between two crownwheels produced from continuously cast bloom rolled to 120mm sq and 140mm dia continuously cast round billet Fourier analysis of the measurements showed a difference of 20ll-m in extreme cases. Improvements in flatness were also seen from the use of a round section. Figure 3b Distortion in Crownwheels from Continuously Cast Round, 140mm dia

Other published work (9) showed an influence of as-cast shape 11m, c in epicyclic annulus gears. 40 An attempt to investigate the effect of as-cast shape. continuously cast round and square and square ingot was made in a collaborative project with BSES, Ovako Steel, Volvo and SKF (10). The cast shapes were produced from one heat to eliminate any hardenability variations as shown below:

90 t on h at Melting (3502) c...-I-...----+---""""""--i....-H-c U -> I -. Continuous Ingot (4.2 ton)

Casting

541mm

+ C Bar rolling

Figure 3a Distortion in Crownwheels from Tube making Continuously Cast Rectangular Bloom, 120mm sq

Component Gear Clutch Cylindrical bearing manufacture blanks sleeves inner rings Figure 4 Steel Manufacturing Route for Cast Shape/Distortion Project

7 Components were produced from the various Rolled shape has had an influence in some cases, but not combinations of cast and rolled shapes in the form of gear all. blanks, bearing rings and gearbox clutch sleeves. Extensive measurements were carried out before and after heat treatment On a worldwide basis, large tonnages of steel for on each of the components. As-cast shape did not have an carburised components are regularly produced in the form of influence upon heat treatment distortion in any of the non-square continuously cast bloom without distortion components. problems being experienced. This includes crownwheels and BSES has been involved in discussions and investigations other critical components. Placed in this context, it has to be with a number of other component producers and a summary concluded that as-cast shape is a relatively minor influencing of this experience, along with some published work is given in factor in heat treatment distortion. Table 1 below.

SOURCE COMPONENT FIXTURE BENFICIAL IF COMPONENT GRADE CASTIROLLED SHAPE SECTION QUENCHED INFLUENCE PUBLISHED OF CAST SHAPE Seger (7) Crownwheel l6MnCr5 CC Slab and CC Sq --> 70mm Sq 25mm+ Yes Yes

Gunnarson (8) Crownwheel INiCr CC l40mm dia, CC Rectangle -->120mm Sq 50mm Yes Yes

Thoden (9) Gearbox Clutch Sleeve 20MoCr4 CC 325mm dia, CC Rectangle -->80mm Sq ? Yes Yes

BSES (10) Gear Blank 1NiCr CC l40mm Sq, CC l60mm dia, Ingot -->100mm dia, 30mm No No 90mmSq

Volvo (10) Gearbox Clutch Sleeve INiCr CC l40mm Sq, CC 160mm dia, Ingot -->100mm dia

SKF (10) BeanngRing INiCr CC 140mm Sq, CC l60mm dia, Ingot --> 100mm dia 7mm No No

Volkmuth(ll) Bearing Ring SAE 52100 CC 325mm dia, CC Rectangle --> 100mm dia 7mm No No

Zoch (12) Bearing Ring SAE 52100 CC Sq, CC Rectangle -->50mm Sq, 6mm Yes No CC 325mm dia --> 45, 55mm dia

Intemal Report Bearing Ring INlCr CC l40mm Sq, CC 160mm dia -->38mm dia 6mm No No .. Crownwheel CrMo CC Rectangle, Ingot -->65mm Sq 30mm No No

" Crownwheel 1NiCr CC 325mm dia, CC Rectangle --> l40mm dia 50mm Yes No CC 140mmdia

" Crownwheel 16MnCr5 CC 325mm dia, 300mm Sq -->65170mm Sq 25mm+ Yes No

" Epicyclic Annulus MoCr Ingot, CC Rectangle -->100mm Sq 20mm+ Yes Yes

" Epicyclic Annulus 1NiCr Ingot, CC Rectangle -->100mm Sq 20mm+ Yes Yes

" Sun Gear lNiCr CC Sq, Rectangle -->76rnn Sq 25mm Yes Yes

Table 1 Investigations into the Effect of As-Cast Shape Upon Distortion

5. Variations in Macrostructure and From Table 1, it can be seen that as-cast shape appears to have had an influence upon distortion of crownwheels and Hardenability epicyclic gear annulus components, made from carburising steels. There are some contradictory results. An extreme example of how the macrostructure of steel Where as-cast shape has been influential the following can influence component properties has been reported by Hock circumstances apply: et al (13). In this case, the use of badly-applied Relatively large section size. electromagnetic stirring below the mould during continuous Fixture or press quenching is applied. casting can give rise to a phenomenon known as "white band". This is shown in Figure 5a. The following comments can be made in these cases: Square or round have given benefits over non-square cast sections. No significant differences between round and square as-cast section have been realised.

8 I A I I Steel mill E I

400 400

=;> =;> -:r: -e;. 300, 300 .,'" .,'" '0..= '0..= :r:0: :r:0: 200 200

70mm I I I Surface Core Surface Core Figure Sa Hardness Traverses on Quenched Billet Sections from Two Sources

45 Jominy Sample

40 light band .. on .. .. "" U .. , l::z:: , a 35 , .. .,'" , '0= '. .. 30 0: :r: Position -e-A 25 ....0l:;Io ... B -A-C ~D " " ""l!\. 20 5 10 25 50 Depth (mm] Figure Sb Jominy Hardenability Variations Across a Billet Section Showing White Band (plots A and B are average values)

The negative and positive segregation effects associated This is an extreme example of variability in the with the white band give rise to significant variations in the macrostructure. Most steelmakers producing engineering hardness across a heat treated billet slice compared to a steels have a good control of this feature but there is inevitably section free of white band, as in Figure 5a. Jominy test pieces a difference in macrostructures from steelmaker to steelmaker, taken across the billet show a significant difference at the reflecting differences in plant and processes. Following the position corresponding to the white band (Figure 5b). In casting and rolling of the steel to billet or bar, components are addition to the potential for "softspots" should the white band produced by forging and/or machining processes. It is the be present at the final component surface, the difference in remnant macrostructure or "grainflow" in these components hardenability is also a potential for variation in distortion prior to heat treatment which can be important. behaviour.

9 Added to the variation in macrostructure across products 2. D.J. Milbourn, "Production and Properties of Modem from different steelmakers is a variation in measured Jominy Engineering Steels", Drives and Controls Conference hardenability - The Jominy hardenability test can be subject to 1998, Telford, UK, 1O-12th March 1998, p 10-15. numerous variables in sampling preparation and testing (14). Whilst most steelmakers achieve a good consistency of 3. D.T. Llewellyn, W.T. Cook, "Heat Treatment Distortion hardenability performance based upon their own results, in Case Carburising Steels", Technology, May significant differences have been shown to exist from 1977, p 265-277. steelmaker to steelmaker against the same nominal specification (6, 13). 4. H. Mallener, "Changes in Dimensions and Shape During In order to overcome potential problems from changes in Case ", AWT Meeting on "Case Hardening", macrostructure and hardenability testing, particularly those of hosted by AWT, Darmstadt, Germany, 12-14th April distortion, some end users (13, 15, 16) source individual or 1989 (in German). families of components against One consistent supply chain ­ The same steelmaker, forger and machinist using the same 5. U. Sabelstrom, S. Carlsson, T. Kvarnstrom, C. Wulliman, consistent process routes. This is known as "monosourcing" S. Andersson, "Influence of Hardenability upon Distortion and is claimed to have significant benefits in the control of and Carburising Time in the Manufacture of Gearbox distortion. Components", Haterei-Technische Mitteilungen, Vol 47 Monosourcing is worthy of further study and application (1992) 4, P 239-244 (in German). for mass-produced carburised components in the automotive industry. 6. D. Rezel and B. Alliet, "Jominy Quench Measurements Performed by Different Laboratories. Comparison of 6. Summary and Conclusions Results Obtained by Measurement and Calculation", 9th International Conference on Heat Treatment and Surface 1. Many of the factors which influence heat treatment Engineering and 5th French Open Conference on Heat distortion are outside the control of the steelmaker. Treatment, Nice, France, 26-28th September 1994, p 401-410 (in French). 2. Of those factors which the steelmaker does control hardenability is the most important. A consisten~ 7. W. Seger, "Influencing the Distortion Behaviour of hardenability is necessary to assist consistent distortion Rotating Symmetrical Forgings by Control of behaviouron heat treatment. Solidification", Munich Seminar on Forming, 25-26th September 1986. Summary published in 3. As-cast shape has been shown to have an influence upon Industrie-Anzeiger 93, 1986, P 22-24 (in German). heat treatment distortion in a limited number of cases. 8. S. Gunnarson "Influence of Continuously Cast Shape 4. The monosourcingof components to reduce variations due upon Distortion of a Case Hardened Steel Crownwheel", to steelmaking, forging and machining operations upon Haterei-Technische Mitteilungen, Vol 46 (No.4) 1991, P distortion behaviour is applied by some producers of 200-216 (in German). mass-produced parts. This is worthy of further investigationand application. 9. B. Thoden, "Dimension Charges and Distortion Resulting from Heat Treatment", AWT Factsheet 15 (in German). 7. Acknowledgements 10. S. Gunnarson, M. Cristinacce, T. Lund and 1. Volkmuth, The author is grateful to Messrs. Gunnarson and "As-Cast Shape Related to Heat Treatment Distortion in Ravenshorst of Volvo AB, Gothenberg, Sweden and Messrs. Circular Shaped Engineering Components", Journal of Beck and Hock of ZF Friedrichshafen, Germany for useful and Engineering Materials and Performance, Vol 4, (3) June informativediscussions on this subject. 1995,p 259-264.

8. References 11. J. Volkmuth, F. Hengerer and T. Lund, "Influence of Casting Method and Shape upon Out of Roundness of 1. E.A. Geary. K. Thomas, P. Avis and D. Bishop, "The Heat Treated Bearing Rings", Haterei-Technische Prediction of Properties in Heat Treated Engineering Mitteilungen, Vol 50 (1995) 6, P 352-358 (in German). Steels", !MechE "Autotech" Conference, Birmingham, UK, 12-15thNovember 1991. 12. H.W. Zoch, T. Lubben, F. Hoffmann and P. Mayr, "Distortion and Continuous Casting - Influence of the Cast Shape upon Fixture Quenching of Bearing Rings",

10 Haterei-Technische Mitteilungen, Vol 49 (1994) 4, P 245-253 (in German).

13. S. Hock, M. Schulz and D. Wiedmann, "Variation in Shape and Dimensions: Some Influencing Parameters and Methods of Prediction", ATTT -AWT Franco-German Days, "Distortion in Heat Treatment", Belfort, France, 24-25th April 1997, p 125-142 (in German).

14. H. Dietrich, W. Schmidt and V. Schuler, "Observation of Errors in the End-Quench (Jominy) Test", Thyssen Edelstahl Techn. Ber. (1984) 2, P 133-144 (in German).

15. T. Lubben, F. Hoffmann, P. Mayr and I. Volkmuth, "A Systematic Approach to Variations in Dimensions and Shape in Heat Treatment", ATTT -AWT Franco-German Days, "Distortion in Heat Treatment", Belfort, France, 24-25th April 1997, p 125-142 (in German).

16. I.e. Le Flour, "Influence of Flow Lines upon Heat Treatment Distortion", ATTT -AWT Franco-German Days. "Distortion in Heat Treatment", Belfort, France. 24-25th April 1997, p119-124 (in French).

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