International Journal of Research in Advent Technology, Vol.2, Issue 4, April 2014 E-ISSN: 2321-9637

Metallurgical Analysis of Cracks Encountered During Induction Hardening of Crankshafts

Mohit Sharma 1, Jasjeet Singh Kohli 2, Shalom Akhai 3

1UG Student, Department of Materials & Metallurgical Engineering, PEC University of Technology (Formerly Punjab Engineering College), Chandigarh, India-160012 2 UG Student, Department of Materials & Metallurgical Engineering, PEC University of Technology (Formerly Punjab Engineering College), Chandigarh, India-160012 3Assistant Professor, Department of Materials & Metallurgical Engineering, PEC University of Technology (Formerly Punjab Engineering College), Chandigarh, India-160012

Email: [email protected] 1

ABSTRACT In this case study, metallurgical analysis of defects encountered during Induction Hardening of crankshafts has been done in detail. Major rejections occur during induction hardening due to cracking of the crankshafts after Induction Hardening/Tempering. Hence, in order to find the root cause of failure & to study the different defects during induction hardening process study was carried out, aim of the case study was to find out reasons behind the cracks, to study the process of induction hardening and the defects encountered during the induction hardening process. A complete failure analysis was done on rejected crankshafts which had developed defects during induction hardening. Different parameters were taken into consideration and various tests were performed to find the reason behind the failure. In the end some corrective actions and suggestions will be provided in order to improve the process and to counter the reasons that cause cracking of crankshafts during induction hardening.

Index Terms - Induction Hardening; Surface Hardness; MPI; Case Depth; Austenite grain size; Stereoscopy; SEM.

1. INTRODUCTION the surface hardening of steel. The components are heated to Induction Hardening process: Induction hardening is a a temperature within or above the transformation range form of heat treatment in which a metal part is heated followed by immediate quenching. The core of the by induction heating and then quenched [3]. The quenched component remains unaffected by the treatment and its metal undergoes a martensitic transformation, increasing physical properties are those of the bar from which it was the hardness and brittleness of the part. Induction hardening machined, whilst the hardness of the case can be within the is used to selectively harden areas of a part or assembly range 37/58 HRC. Carbon and alloy steels with an without affecting the properties of the part as a whole. The equivalent carbon content in the range 0.40/0.45% are most generated electric current in varying magnetic fields known suitable for this process [1] . as “EDDY CURRENT”. These eddy currents generated are responsible for the heating up of the work piece. 2. LITERATURE REVIEW Induction heating is a non contact heating process which 2.1 Induction Hardening parameters utilizes the principle of electromagnetic induction to produce • Frequency heat inside the surface layer of a work-piece. By placing • Power consumption a conductive material into a strong alternating magnetic • Heating time field, electrical current can be made to flow in the material • Voltage thereby creating heat due to the I2R losses in the material. • Current The current generated flows predominantly in the surface layer, the depth of this layer being dictated by the frequency of the alternating field, the surface power density, the permeability of the material, the heat time and the diameter of the bar or material thickness. By quenching this heated layer in water, oil or a polymer based quench the 2.2 Defects during induction hardening surface layer is altered to form a martensitic structure which Different types of defects are encountered during induction is harder than the base metal It is a widely used process for hardening, such as:

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• Overheating and burning of steels : Occur due to quenchant have widely differing properties. Great flexibility of selection of wrong power and time settings. Dimension quenching characteristics is possible through selection of the of the job and the material and the final product should type of polymer, polymer concentration, temperature of the bath be kept in mind while setting the power and time and degree of agitation. parameters to avoid overheating and burning of steels. • Cracks/Quench Cracks : Very high cooling rate results Several different types of organic polymers [17] are used in cracks, low concentration of quenchant and high including: severity can result in quench cracks. • PAG Polyalkylene glycol • Internal residual stresses : Due to expansion and • ACR Sodium polyacrylate contraction during the process, stresses are generated in • PVP Polyvinyl pyrrolidone the job that can harm it at later stages and under working • PEO Polyethyloxazoline. conditions. • Dimensional changes : It also occurs due to the 2.3.2 Technical Advantages expansion and contraction of the material during the 1. Flexibility of quenching speed : By varying the heating and cooling of the job and during the formation concentration, temperature and agitation of the polymer of martensite. solution, it is possible to achieve a range of cooling • Soft spots : these spots of lower hardness can result due rates, thus enabling the treatment of a wide variety of to improper agitation of the quenchant and due to leftover materials and components. lubricants and other impurities on the surface of the job. 2. Elimination of soft spots : By producing a uniform Proper cleaning of the job before hardening and proper polymer film around the component, the steam agitation of the bath are necessary to avoid soft spots. pocketing and soft spot problems often associated with • Low hardness : improper temperature selection, quench water quenching after induction hardening can be severity and hardening time result in lower hardness than avoided. required. Geometry and material should be studied 3. Reduction of stresses and distortion: The uniform properly in order to set the above said parameters to film also reduces thermal gradients and residual stresses achieve desired hardness level. associated with water quenching and can, therefore, give substantial reduction in distortion during the solution 2.3 Quenching heat treatment of aluminum alloys. Quenching is the rapid cooling of a work piece to obtain 4. Tolerance to water contamination : Large amounts of certain material properties [3] . In metallurgy, it is most water contamination can be tolerated before commonly used to harden steel by introducing martensite, in concentration (and hence quenching speed) is influenced which case the steel must be rapidly cooled through its significantly. This eliminates the soft spot, distortion eutectoid point, the temperature at which austenite becomes and cracking. unstable. 2.3.3 Production Advantages 2.3.1 Types of quenchants 1. Reduced cost: Depending upon the type of polymer Quenchants: Selection of quenchant depends on many and the concentration required, the in-tank costs of factors, such as: diluted polymer quenchants can be considerably lower • Cooling rate required than those of quenching oils. Because polymer • Hardness required solutions have significantly lower viscosities than • Material properties i.e. hardenability quenching oils, drag-out and hence replenishment requirements are reduced. A number of quenchants available today are: 2. Easier cleaning: Components may not require • Gases cleaning before tempering. Residual films of polymers • Oil that may contain a variety of additives will not char, as with oils, but will decompose fully at high temperatures to form water vapor and oxides of • Water carbon. Components may be tempered directly after • Aqueous polymer solutions • quenching, thereby eliminating the need for costly Water that may contain salt or caustic additives alkali cleaning or vapor degreasing operations. Out of these mentioned quenchants, polymers are the most 3. Reduced temperature rise during quenching: widely used quenchant in industries. Polymer quenchant solution have almost twice the Polymer quenchants consist of solutions of organic polymers in specific heat capacity to that of quenching oils. water and contain corrosion inhibitors and other additives to Therefore, for a given charge weight, the temperature produce concentrates, which are further diluted to give ready-to- rise during quenching will be approximately halved. use quenching solutions. The various types of polymer

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Most widely used polymer quenchant in the industry today Micro-indentation hardness test, hardness test using a are based on PAG (Polyalkylene Glycol). calibrated machine to force a diamond indenter of specific Commercial PAG based quenchants also contain a number geometry into the surface of the material being evaluated, in of additives such as corrosion inhibitors, defoamers and which the test forces are 9.807 3 10-3 to 9.807 N (1 to 1000 bactericides to enhance performance in service. gf) [11] and the indentation diagonal, or diagonals are measured with a light microscope after load removal; for any 2.3.4 Degradation Of Quenchant Polymers test, it is assumed that the indentation does not undergo Polymer quenchants may degrade by a number of potential elastic recovery after force removal [12] . The test results are mechanisms [18] . Three of the most common degradation normally in the KNOOP or VICKERS scales. processes are: VICKER HARDNESS TESTER. In this test method, a 1. Biological degradation Vickers hardness number is determined based on the 2. Mechanodegradation formation of a relatively small indentation made in the test 3. Thermal/oxidative degradation surface of samples being evaluated. A Vickers indenter, Polymer degradation may be detected and quantified by a made from diamond of specific geometry (a square-based direct analysis of the change in the size of the polymer by a pyramidal-shaped diamond indenter with face angles of classical technique such as SEC (size exclusion 136°), is pressed into the test specimen surface by an chromatography). accurately controlled applied force using test machines specifically designed for such work. 2.4 Magnetic Particle Inspection The Vickers Hardness No. (VHN) is measured with the help The Magnetic Particle Inspection (MPI) method (Non- of the following formula, destructive type) is applicable for detecting surface and sub- VHN = 1854.4 * (P/d2) surface defects, which are not visible to naked eye. Where, The magnetic particle method is based on the PRINCIPLE P= load in gf that ― Magnetic field lines when present in a ferromagnetic d= mean diagonal length of indentation in microns material will be distorted by a change in material continuity, such as a sharp dimensional change or a discontinuity [14] . If the 2.7 Metallographic Evaluation (Micro structural Analysis) discontinuity is open to or close to the surface of a magnetized Contrast Enhancement and Etching - A polished specimen material, flux lines will be distorted at the surface, a condition frequently will not exhibit its microstructure, because light is termed as ―flux leakage. When fine magnetic particles are uniformly reflected [13] [6] . The eye cannot discern small distributed over the area of the discontinuity while the flux differences in reflectivity; therefore, image contrast must be leakage exists, they will be held in place and the accumulation produced. Metallographic contrasting methods include of particles will be visible under the proper lighting conditions. various electrochemical, optical, and physical etching techniques. Commonly used chemical etchant is 3% Nital. 2.5 The Spectral Analysis Light Microscopy – Except inclusion, other characteristics Technique is based on the principle of EDS, i.e., Electron like grain size, decarburization etc are better viewed after Dispersive Spectroscopy, which is carried out using a SEM etching. (Scanning Electron Microscope). As high-energy electrons produced with an SEM interact with the atoms within the top 2.8 Austenite Grain Size Measurement few micrometers of a specimen surface, X-rays are generated The sizes of the martensite plates are determined by the with an energy characteristic of the atom that produced austenite grain size and the continuous formation of them. The intensity of such X-rays is proportional to the additional plates with decreasing temperature [4]. The first mass fraction of that element in the specimen. In energy- plates form at the martensite start (Ms) temperature and dispersive spectroscopy, X-rays from the specimen are spanthe austenite grains; the longest dimension of these detected by a solid-state spectrometer that converts them to plates is therefore equivalent to the austenitic grain size. electrical pulses proportional to the characteristic X-ray With decreasing temperature, more martensite plates form energies. If the X-ray intensity of each element is compared between large plates, and they become finer as the austenite to that of a standard of known composition and suitably is increasingly partitioned by more transformation. corrected for the effects of other elements present, then the According to this sequence of martensite formation, the finer mass fraction of each element can be calculated. the austenite grain size, the finer is the array of martensite Trace elements, defined as <1.0%, can be analyzed but with plates. lower precision compared with analyses of elements present The final austenite grain size, which controls the size of the in greater concentration. martensite plates and laths, is smaller, and therefore the 2.6 Microhardness Testing microstructure is more refined.

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A study of the factors that control the resistance to brittle optical metallograph. Scanning electron microscope images fracture in these steels indicated the importance of the prior are monochromatic and, as such, do not benefit by color austenite grain size [10] . photography. The primary techniques of photography Large grain size indicates high heating temperature or associated with scanning electron microscopy are digital overheating. And overheating leads to higher temperature photography and instant film photography. X-ray imaging gradient when the job is quenched and thus may lead to spectroscopy associated with scanning electron microscopy cracking of the job due to high temperature gradient. typically uses digital photography for the recording of x-ray To measure the austenite grain size, the steps followed were: images and maps. Videographic cameras may also be used to Carburizing of the sample at 950 ºC record entire scanning electron microscopy examination Oxidation of grain boundaries sessions Roughly polish the sample Study microstructure under the microscope and 3. METHODOLOGY measurement of the grain size by comparison method. Steps Involved in the Metallurgical Investigation [5] [20] 1. Chemical Composition Analysis 2.9 Crack Opening and Stereography 2. Induction Hardening Process Details Optical stereomicroscopy is routinely used in failure analysis 3. Magnetic Particle Inspection photography to examine and characterize fracture surface 4. Surface Hardness features [5] . The primary techniques for stereomicroscopy photography is film photography, instant film photography, 5. Effective Case Depth measurement digital photography, and, in some applications, videography. 6. Austenite Grain size measurement In Stereography images upto a magnification of 50X are 7. Polymer quenchant analysis studied. 8. Stereoscopy Optical metallography is used in failure analysis 9. SEM Analysis photography to characterize the macro- and microstructure of a failed component. 3.1 Chemical Composition Optical metallography has historically used monochromatic Material of crankshaft manufactured: lighting and black-and-white photography. However, color Grade: SAE 1548 photography is advantageous for showing heating effects, Material: micro alloyed steel corrosion product, and microstructural features with color. In Material Composition: stereography, the magnified images are studied and the above said features can be studied in detail ELEMENT PERCENTAGE ACTUAL Carbon 0.44 – 0.50 0.45 2.10 SEM Analysis THE SCANNING ELECTRON MICROSCOPE (SEM) is Manganese 1.10 – 1.40 1.16 one of the most versatile instruments for investigating the microstructure of metallic materials. Compared to the optical Silicon 0.20 – 0.35 0.25 microscope, it expands the resolution range by more than one order of magnitude to approximately 10 nm in routine Phosphorous 0.025 0.014 instruments, with ultimate values below 3 nm. Useful Sulphur 0.20 - 0.005 0.011 magnification thus extends beyond 10,000× up to 100,000×, closing the gap between the optical and the transmission Chromium 0.10 – 0.20 0.14 electron microscope. Compared to optical microscopy, the depth of focus, ranging from 1 µm at 10,000× to 2 mm (0.08 Nickel 0.10 Max. 0.040 in.) at 10×, is larger by more than two orders of magnitude. Scanning electron microscopy is used in failure analysis Molybdenum 0.060 Max. 0.010 photography for the examination of surfaces, including Aluminium 0.02 – 0.04 0.024 fracture surfaces, and the recording of fracture features. Scanning electron microscopy is used in failure analysis Copper 0.080 Max. 0.020 photography for the examination of fracture surfaces and other surface features and the recording of features present, Table 1: comparison of the actual composition of the crankshaft material to as well as high magnification recording of the the standard material microstructure. The scanning electron microscope is a higher-magnification extension of the optical stereomicroscope as well as the

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Remarks: The composition was found to be appropriate and 3.2 Induction Hardening Process Details according to the specification of the customer. Therefore, no Machine make: IEI chemical composition is accurate. Capacity: 225 KW Frequency: 10 KHz (medium frequency) Freq Quench Quench Location Inductor POT KW V (v) I (amp) Time (s) (KHz) Delay time Pin B234/3 88/80 175/135 700/640 600/580 10.5/10 17 0.5 s 20 s J1 B232/3 90 185 720 640 10 17 0.5 s 20 s OJ B233/3 90 185 720 640 10 17 0.5 s 20 s J4,7 B233/3 90 185 720 640 10 18 0.5 s 20 s

Table 2 : Process Parameters of the induction hardening machine used for the process

Remarks: The parameters were cross checked with the actual parameters that were being used on the induction hardening machine and were found to be appropriate. Therefore, process parameters are not the problem.

3.3 Magnetic Particle Inspection (MPI) MPI machine : Company : Vinze. Curren t Type: DC. Current applied: 1600 Amp max.

Magnetic solution : Ferro flux (0.3gm/cc) + kerosene 3.4 Surface Hardness Specification: 45 – 50 HRC Min. Location Pin1 Pin2 Pin3 Pin4 Pin5 Pin 6 Avg

Cracked 49.9 49.6 49.8 49.5 50 49.9 49.7 sample

Un Cracked 49.3 49.8 49.5 49.7 49.9 49.8 49 .6 sample

Table 3: Surface hardness comparison of pins of cracked and uncracked shafts

Remarks: No variation observed in surface hardness of cracked & un-cracked crankshaft. Core Hardness: Specification 212 - 277 HBW

Cracked sample 229 – 269 Un Cracked sample 231 – 269 Photo1: Arrow indicating Circular Open crack at Pin 2 Side Face

Table 4: core hardness of cracked and uncracked samples

Remarks : No variation observed in core hardness of cracked & un-cracked crankshaft

3.5 Effective Case Depth Measurement Microhardness Company : Clemex Load Range : 1gm to 2000gm: Load used : 500gm Dwell Time : 13 seconds: Transverse distance : 0.5mm

Photo2: Arrow indicating Circular Open crack at Pin 5 Side Face

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Photo 3: Visual case depth measurement by etching the cut sample and measuring by vernier calipers

Cracked sample Un Cracked sample

Location Diameter Fillet GE Fillet FE Diameter Fillet GE Fillet FE Spec. in mm 2.03 min 1.27 min 2.03 min 1.27 min Top 4.26 3.17 3.12 4.52 3.46 3.32 Pin 5 Bottom 4.60 3.32 3.28 4.71 3.58 3.45

Table 4: Case depth measurements of cracked and uncracked samples at different positions.

Remarks : No variation observed in effective case depth of cracked & un-cracked crankshaft. Photo 5: Austenite grain size of the un-cracked sample. ASTM Grain 3.6 Austenite Grain Size Measurement [15] size: 7 – 8 checked as per ASTM E-112 method of comparison

Note: The above photos were taken at higher magnification (500X) to check the size of the individual grain. Remarks : No variation observed austenite grain size of cracked & un-cracked crankshaft

3.7 Analysis of Polymer Quenchant Name: Polydur AL Company: Houghton

Concentration: 14% Photo 4: Austenite grain size of the cracked sample. ASTM Grain size: Quench flow rate: 3.0 kg/cm2 7 – 8 checked as per ASTM E-112 method of comparison Bath Temperature: 34-360C

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Figure 1: Cooling curve of Ideal Quench Bath with 9% Polymer concentration.

Figure 2: Cooling curve of Quench Bath with 12% polymer concentration (Quench cracks occurred with this bath).

The table given above table shows the actual quench bath parameters at 12% concentration. The cooling rate at 300º C is 70.710C/sec. 300º C is taken as reference temperature as martensite begins to form at this temperature. But, the ideal cooling rate at this concentration and at 300º C should be between 40-50º C/sec. This shows that the actual cooling rate is higher than the ideal cooling rate. The reason behind this could be bacterial degradation of polymer quenchant and contamination.

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3.8 Stereography (after crack opening) The crack was opened and cleaned ultrasonically to prepare it for stereographic analysis. Microscope: Ziess Maximum magnification : 50 X Magnification used : 10X and 25X Sample after crack opening was done. The two surfaces seen were cleaned ultrasonically to view them under the microscope Photo 7: Sample under stereoscope after crack opening at 10X

Photo 6: Sample after crack opening ready for stereography

Photo 8: Stereoscope image at 25X, oxidation of inner surface can be seen after crack opening.

Remarks: Stereoscopic study reveals the details of the nature of crack origin point & magnifies fracture surface.

3.9 SEM Analysis Company : ZEISS Maximum Magnification : 1000000X Magnification Range Used : 500-3500X Electron Beam Source : LaB6 SEM Results: The sample after stereoscopy was analyzed under the SEM. The crack surface was cleaned ultrasonically and dried and then analyzed under the SEM at various magnifications. The images taken from the SEM are as shown below:

Photo 10: SEM images of fractu re surface at 500X

Photo 9: SEM images of fracture surface at 150 X magnification showing crack initiation point .

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Inclusions, as the SEM images don’t show presence of any such inclusions.

CRACK ANALYSIS : Type of crack: QUENCH CRACK This is indicated by the stereography and the SEM & EDS analysis. The nature of crack observed in the crankshaft is similar to quench crack & also presence of FeO/ Fe 2O3 layer on the fracture surface .

Reason behind the cracking during Induction Hardening : Comparison of actual quench bath parameters to ideal bath parameters: The comparison shows that the actual Photo 11: SEM images of fracture surface at 500 X showing Intergranular cracks in the sample. bath parameter i.e. the cooling rate, deviate a lot from the ideal parameters. This indicates that there is a problem related to the bath parameters. The only reason that can be behind the abnormal behavior of the bath is: degradation of the polymer which could be due to bacterial attack or contamination of the bath. The degradation results in increase in quench severity and hence an increase in cooling rate, this leads to quench cracks during induction hardening of the crankshafts.

5. CONCLUSION AND FUTURE WORK Based on the analysis of the above mentioned tests and data, and the conclusion drawn by them, some solutions can be suggested to improve the process and to reduce the cracks encountered during the induction Figure 3: EDS analysis results showing presence of C, O, Fe, S, Mn in hardening. Some of them are: the tested sample. 1. Periodic quenchant analysis: As the performance of the polymer quenchant is dependent on the condition EDS Analysis Remarks: EDS analysis confirms of the polymer and potential contamination, it is presence of FeO/ Fe O layer on the fracture surface.. 2 3 important that periodic maintenance be performed.

Some of the tests performed include: refractive index, SEM & EDS analysis shows the following: viscosity, separation temperature, conductance, 1. Intergranular cracks corrosion inhibitor concentration and, if needed, foam 2. Brittle fracture tests. In some cases, cooling curve analyses are 3. Crack extend from surface to the center performed. 4. Presence of FeO/ Fe2O3 layer. 2. The quenchant must be kept clean and free from These types of characteristics are shown typically by contamination. Solid contamination not only produces QUENCH CRACKS . Therefore, it can be concluded non-uniform vapour blanket at the hot metal interface that the crack is generated during quenching of but it also plugs quench holes. Induction hardening process. The presence of FeO/ 3. Increase the polymer concentration from 12% to Fe O layer on the fracture surface developed during 2 3 14% to compensate the effect of polymer degradation. tempering also confirms cracking occurred during Further improvements in the process can be made by induction hardening. altering some other parameters and factors such as

changing the polymer quenchant, addition of additives 4. RESULTS change in quench pressure etc. All the above said tests were carried out and the data is collected and analyzed. The conclusions that were REFERENCES : reached: [1] ASM Handbook Volume 1 - Properties and Reasons behind cracks that can be eliminated: Selection Overheating, as ASTM grain size of both the samples [2] Irons Steels and High Performance Alloys is same.

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[3] ASM Handbook Volume 4 – Heat Treating [4] ASM Handbook Volume 9 – Metallography [5] ASM Handbook Volume 11 - Failure Analysis ASTM STANDARDS [6] E3 - Preparation of Metallographic Samples [7] E7-Standard Terminology Relating to Metallography [8] E 10 – Brinell hardness of Metallic Materials [9] E 18 – Rockwell Hardness and Rockwell Superficial [] Hardness of Metallic Materials [10] E112 – Determining Average Grain Size [11] E384 – Micro indentation Hardness method [12] E407 – Micro-etching Metals and Alloys [13] E807 – Metallographic Laboratory Evaluation [14] E1444 - Standard Practice for Magnetic Particle Testing [15] E112 – Determining Average Grain Size [16] E384 – Micro indentation Hardness method [17] Totten G.E.; “Polymer quenchants for induction heat treating applications: the basics”. [18] Dow Corning; “Degradation of polymers in nature”; HERA [19] Totten G.E. & Webster G.M.; “Importance of quench bath maintenance”. [20] C. Kendall Clarke & Don Halimunanda; “Failure analysis of induction hardened automotive axles”; ASM International, May 2008.

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