Metallurgical Analysis of Cracks Encountered During ... id-242014142.pdf · Metallurgical Analysis...

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

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Metallurgical Analysis of Cracks Encountered During Induction Hardening of Crankshafts

Mohit Sharma 1, Jasjeet Singh Kohli2, Shalom Akhai3

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]

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 Induction Hardening process: Induction hardening is a form of heat treatment in which a metal part is heated by induction heating and then quenched [3]. The quenched metal undergoes a martensitic transformation, increasing the hardness and brittleness of the part. Induction hardening is used to selectively harden areas of a part or assembly without affecting the properties of the part as a whole. The generated electric current in varying magnetic fields known as “EDDY CURRENT”. These eddy currents generated are responsible for the heating up of the work piece. Induction heating is a non contact heating process which utilizes the principle of electromagnetic induction to produce heat inside the surface layer of a work-piece. By placing a conductive material into a strong alternating magnetic field, electrical current can be made to flow in the material thereby creating heat due to the I2R losses in the material. 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 surface layer is altered to form a martensitic structure which is harder than the base metal It is a widely used process for

the surface hardening of steel. The components are heated to a temperature within or above the transformation range followed by immediate quenching. The core of the component remains unaffected by the treatment and its physical properties are those of the bar from which it was machined, whilst the hardness of the case can be within the range 37/58 HRC. Carbon and alloy steels with an equivalent carbon content in the range 0.40/0.45% are most suitable for this process [1]. 2. LITERATURE REVIEW 2.1 Induction Hardening parameters

• Frequency • Power consumption • Heating time • Voltage • Current

2.2 Defects during induction hardening Different types of defects are encountered during induction hardening, such as:


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• Overheating and burning of steels: Occur due to selection of wrong power and time settings. Dimension of the job and the material and the final product should be kept in mind while setting the power and time parameters to avoid overheating and burning of steels.

• Cracks/Quench Cracks: Very high cooling rate results in cracks, low concentration of quenchant and high severity can result in quench cracks.

• Internal residual stresses: Due to expansion and contraction during the process, stresses are generated in the job that can harm it at later stages and under working conditions.

• Dimensional changes: It also occurs due to the expansion and contraction of the material during the heating and cooling of the job and during the formation of martensite.

• Soft spots: these spots of lower hardness can result due to improper agitation of the quenchant and due to leftover lubricants and other impurities on the surface of the job. Proper cleaning of the job before hardening and proper agitation of the bath are necessary to avoid soft spots.

• Low hardness: improper temperature selection, quench severity and hardening time result in lower hardness than required. Geometry and material should be studied properly in order to set the above said parameters to achieve desired hardness level.

2.3 Quenching Quenching is the rapid cooling of a work piece to obtain certain material properties [3]. In metallurgy, it is most commonly used to harden steel by introducing martensite, in which case the steel must be rapidly cooled through its eutectoid point, the temperature at which austenite becomes unstable. 2.3.1 Types of quenchants Quenchants: Selection of quenchant depends on many factors, such as: • Cooling rate required • Hardness required • Material properties i.e. hardenability

A number of quenchants available today are:

• Gases • Oil that may contain a variety of additives • Water • Aqueous polymer solutions • Water that may contain salt or caustic additives

Out of these mentioned quenchants, polymers are the most widely used quenchant in industries. Polymer quenchants consist of solutions of organic polymers in water and contain corrosion inhibitors and other additives to produce concentrates, which are further diluted to give ready-to-use quenching solutions. The various types of polymer

quenchant have widely differing properties. Great flexibility of quenching characteristics is possible through selection of the type of polymer, polymer concentration, temperature of the bath and degree of agitation.

Several different types of organic polymers [17] are used including:

• PAG Polyalkylene glycol • ACR Sodium polyacrylate • PVP Polyvinyl pyrrolidone • PEO Polyethyloxazoline.

2.3.2 Technical Advantages 1. Flexibility of quenching speed: By varying the

concentration, temperature and agitation of the polymer solution, it is possible to achieve a range of cooling rates, thus enabling the treatment of a wide variety of materials and components.

2. Elimination of soft spots: By producing a uniform polymer film around the component, the steam pocketing and soft spot problems often associated with water quenching after induction hardening can be avoided.

3. Reduction of stresses and distortion: The uniform film also reduces thermal gradients and residual stresses associated with water quenching and can, therefore, give substantial reduction in distortion during the solution heat treatment of aluminum alloys.

4. Tolerance to water contamination: Large amounts of water contamination can be tolerated before concentration (and hence quenching speed) is influenced significantly. This eliminates the soft spot, distortion and cracking.

2.3.3 Production Advantages 1. Reduced cost: Depending upon the type of polymer

and the concentration required, the in-tank costs of diluted polymer quenchants can be considerably lower than those of quenching oils. Because polymer solutions have significantly lower viscosities than quenching oils, drag-out and hence replenishment requirements are reduced.

2. Easier cleaning: Components may not require cleaning before tempering. Residual films of polymers will not char, as with oils, but will decompose fully at high temperatures to form water vapor and oxides of carbon. Components may be tempered directly after quenching, thereby eliminating the need for costly alkali cleaning or vapor degreasing operations.

3. Reduced temperature rise during quenching: Polymer quenchant solution have almost twice the specific heat capacity to that of quenching oils. Therefore, for a given charge weight, the temperature rise during quenching will be approximately halved.


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Most widely used polymer quenchant in the industry today are based on PAG (Polyalkylene Glycol). Commercial PAG based quenchants also contain a number of additives such as corrosion inhibitors, defoamers and bactericides to enhance performance in service.

2.3.4 Degradation Of Quenchant Polymers Polymer quenchants may degrade by a number of potential mechanisms [18]. Three of the most common degradation processes are: 1. Biological degradation 2. Mechanodegradation 3. Thermal/oxidative degradation

Polymer degradation may be detected and quantified by a direct analysis of the change in the size of the polymer by a classical technique such as SEC (size exclusion chromatography).

2.4 Magnetic Particle Inspection The Magnetic Particle Inspection (MPI) method (Non-

destructive type) is applicable for detecting surface and sub-surface defects, which are not visible to naked eye.

The magnetic particle method is based on the PRINCIPLE that ― Magnetic field lines when present in a ferromagnetic material will be distorted by a change in material continuity, such as a sharp dimensional change or a discontinuity [14]. If the discontinuity is open to or close to the surface of a magnetized material, flux lines will be distorted at the surface, a condition termed as ―flux leakage. When fine magnetic particles are distributed over the area of the discontinuity while the flux leakage exists, they will be held in place and the accumulation of particles will be visible under the proper lighting conditions.

2.5 The Spectral Analysis

Technique is based on the principle of EDS, i.e., Electron Dispersive Spectroscopy, which is carried out using a SEM (Scanning Electron Microscope). As high-energy electrons produced with an SEM interact with the atoms within the top few micrometers of a specimen surface, X-rays are generated with an energy characteristic of the atom that produced them. The intensity of such X-rays is proportional to the mass fraction of that element in the specimen. In energy-dispersive spectroscopy, X-rays from the specimen are detected by a solid-state spectrometer that converts them to electrical pulses proportional to the characteristic X-ray energies. If the X-ray intensity of each element is compared to that of a standard of known composition and suitably corrected for the effects of other elements present, then the mass fraction of each element can be calculated. Trace elements, defined as <1.0%, can be analyzed but with lower precision compared with analyses of elements present in greater concentration. 2.6 Microhardness Testing

Micro-indentation hardness test, hardness test using a calibrated machine to force a diamond indenter of specific geometry into the surface of the material being evaluated, in which the test forces are 9.807 3 10-3 to 9.807 N (1 to 1000 gf) [11] and the indentation diagonal, or diagonals are measured with a light microscope after load removal; for any test, it is assumed that the indentation does not undergo elastic recovery after force removal [12]. The test results are normally in the KNOOP or VICKERS scales. VICKER HARDNESS TESTER. In this test method, a Vickers hardness number is determined based on the formation of a relatively small indentation made in the test surface of samples being evaluated. A Vickers indenter, made from diamond of specific geometry (a square-based pyramidal-shaped diamond indenter with face angles of 136°), is pressed into the test specimen surface by an accurately controlled applied force using test machines specifically designed for such work. The Vickers Hardness No. (VHN) is measured with the help of the following formula, VHN = 1854.4 * (P/d2) Where, P= load in gf d= mean diagonal length of indentation in microns 2.7 Metallographic Evaluation (Micro structural Analysis) Contrast Enhancement and Etching - A polished specimen frequently will not exhibit its microstructure, because light is uniformly reflected [13] [6]. The eye cannot discern small differences in reflectivity; therefore, image contrast must be produced. Metallographic contrasting methods include various electrochemical, optical, and physical etching techniques. Commonly used chemical etchant is 3% Nital. Light Microscopy – Except inclusion, other characteristics like grain size, decarburization etc are better viewed after etching.

2.8 Austenite Grain Size Measurement The sizes of the martensite plates are determined by the austenite grain size and the continuous formation of additional plates with decreasing temperature [4]. The first plates form at the martensite start (Ms) temperature and spanthe austenite grains; the longest dimension of these plates is therefore equivalent to the austenitic grain size. With decreasing temperature, more martensite plates form between large plates, and they become finer as the austenite is increasingly partitioned by more transformation. According to this sequence of martensite formation, the finer the austenite grain size, the finer is the array of martensite plates. The final austenite grain size, which controls the size of the martensite plates and laths, is smaller, and therefore the microstructure is more refined.


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A study of the factors that control the resistance to brittle fracture in these steels indicated the importance of the prior austenite grain size [10]. Large grain size indicates high heating temperature or overheating. And overheating leads to higher temperature gradient when the job is quenched and thus may lead to cracking of the job due to high temperature gradient. To measure the austenite grain size, the steps followed were: Carburizing of the sample at 950 ºC Oxidation of grain boundaries Roughly polish the sample Study microstructure under the microscope and measurement of the grain size by comparison method. 2.9 Crack Opening and Stereography Optical stereomicroscopy is routinely used in failure analysis photography to examine and characterize fracture surface features [5]. The primary techniques for stereomicroscopy photography is film photography, instant film photography, digital photography, and, in some applications, videography. In Stereography images upto a magnification of 50X are studied. Optical metallography is used in failure analysis photography to characterize the macro- and microstructure of a failed component. Optical metallography has historically used monochromatic lighting and black-and-white photography. However, color photography is advantageous for showing heating effects, corrosion product, and microstructural features with color. In stereography, the magnified images are studied and the above said features can be studied in detail 2.10 SEM Analysis THE SCANNING ELECTRON MICROSCOPE (SEM) is one of the most versatile instruments for investigating the microstructure of metallic materials. Compared to the optical microscope, it expands the resolution range by more than one order of magnitude to approximately 10 nm in routine instruments, with ultimate values below 3 nm. Useful magnification thus extends beyond 10,000× up to 100,000×, closing the gap between the optical and the transmission electron microscope. Compared to optical microscopy, the depth of focus, ranging from 1 µm at 10,000× to 2 mm (0.08 in.) at 10×, is larger by more than two orders of magnitude. Scanning electron microscopy is used in failure analysis photography for the examination of surfaces, including fracture surfaces, and the recording of fracture features. Scanning electron microscopy is used in failure analysis photography for the examination of fracture surfaces and other surface features and the recording of features present, as well as high magnification recording of the microstructure. The scanning electron microscope is a higher-magnification extension of the optical stereomicroscope as well as the

optical metallograph. Scanning electron microscope images are monochromatic and, as such, do not benefit by color photography. The primary techniques of photography associated with scanning electron microscopy are digital photography and instant film photography. X-ray imaging spectroscopy associated with scanning electron microscopy typically uses digital photography for the recording of x-ray images and maps. Videographic cameras may also be used to record entire scanning electron microscopy examination sessions 3. METHODOLOGY Steps Involved in the Metallurgical Investigation [5] [20]

1. Chemical Composition Analysis 2. Induction Hardening Process Details 3. Magnetic Particle Inspection 4. Surface Hardness 5. Effective Case Depth measurement 6. Austenite Grain size measurement 7. Polymer quenchant analysis 8. Stereoscopy 9. SEM Analysis

3.1 Chemical Composition Material of crankshaft manufactured: Grade: SAE 1548 Material: micro alloyed steel Material Composition:

ELEMENT PERCENTAGE ACTUAL

Carbon 0.44 – 0.50 0.45

Manganese 1.10 – 1.40 1.16

Silicon 0.20 – 0.35 0.25

Phosphorous 0.025 0.014

Sulphur 0.20 - 0.005 0.011

Chromium 0.10 – 0.20 0.14

Nickel 0.10 Max. 0.040

Molybdenum 0.060 Max. 0.010

Aluminium 0.02 – 0.04 0.024

Copper 0.080 Max. 0.020

Table 1: comparison of the actual composition of the crankshaft material to

the standard material


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Remarks: The composition was found to be appropriate and according to the specification of the customer. Therefore, no chemical composition is accurate.

3.2 Induction Hardening Process Details Machine make: IEI Capacity: 225 KW Frequency: 10 KHz (medium frequency)

Location Inductor POT KW V (v) I (amp) Freq

(KHz) Time (s)

Quench Delay

Quench 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. Current Type: DC. Current applied: 1600 Amp max. Magnetic solution: Ferro flux (0.3gm/cc) + kerosene

Photo1: Arrow indicating Circular Open crack at Pin 2 Side Face

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

3.4 Surface Hardness Specification: 45 – 50 HRC Min.

Location Pin1 Pin2 Pin3 Pin4 Pin5 Pin 6 Avg

Cracked sample

49.9 49.6 49.8 49.5 50 49.9 49.7

Un Cracked sample

49.3 49.8 49.5 49.7 49.9 49.8 49 .6

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

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


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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

Pin 5 Top 4.26 3.17 3.12 4.52 3.46 3.32

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. 3.6 Austenite Grain Size Measurement [15]

Photo 4: Austenite grain size of the cracked sample. ASTM Grain size: 7 – 8 checked as per ASTM E-112 method of comparison

Photo 5: Austenite grain size of the un-cracked sample. ASTM Grain 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% Quench flow rate: 3.0 kg/cm2 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.

International Journal of

3.8 Stereography (after crack opening) The crack was opened and cleaned ultrasonically toprepare 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 6: Sample after crack opening ready for stereography

3.9 SEM Analysis Company: ZEISS Maximum Magnification: 1000000X Magnification Range Used: 500-3500XElectron 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 9: SEM images of fracture surface at 150 X magnification showing crack initiation point

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

The crack was opened and cleaned ultrasonically to

Sample after crack opening was done. The two surfaces seen were cleaned ultrasonically to view them under the

after crack opening ready for stereography

Photo 7: Sample under stereoscope after crack opening at 10X

Photo 8: Stereoscope image at seen after crack opening.

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

3500X

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

surface at 150 X

magnification showing crack initiation point.

Photo 10: SEM images of fractu

Research in Advent Technology, Vol.2, Issue 4, April 2014

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stereoscope after crack opening at 10X

Stereoscope image at 25X, oxidation of inner surface can be

seen after crack opening.

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

SEM images of fracture surface at 500X


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Photo 11: SEM images of fracture surface at 500 X showing Intergranular cracks in the sample.

Figure 3: EDS analysis results showing presence of C, O, Fe, S, Mn in

the tested sample.

EDS Analysis Remarks: EDS analysis confirms presence of FeO/ Fe2O3 layer on the fracture surface.. SEM & EDS analysis shows the following:

1. Intergranular cracks 2. Brittle fracture 3. Crack extend from surface to the center 4. Presence of FeO/ Fe2O3 layer.

These types of characteristics are shown typically by QUENCH CRACKS. Therefore, it can be concluded that the crack is generated during quenching of Induction hardening process. The presence of FeO/ Fe2O3 layer on the fracture surface developed during tempering also confirms cracking occurred during induction hardening.

4. RESULTS All the above said tests were carried out and the data is collected and analyzed. The conclusions that were reached: Reasons behind cracks that can be eliminated: Overheating, as ASTM grain size of both the samples is same.

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/ Fe2O3 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 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 hardening. Some of them are: 1. Periodic quenchant analysis: As the performance of the polymer quenchant is dependent on the condition of the polymer and potential contamination, it is important that periodic maintenance be performed. Some of the tests performed include: refractive index, viscosity, separation temperature, conductance, corrosion inhibitor concentration and, if needed, foam tests. In some cases, cooling curve analyses are performed. 2. The quenchant must be kept clean and free from contamination. Solid contamination not only produces non-uniform vapour blanket at the hot metal interface but it also plugs quench holes. 3. Increase the polymer concentration from 12% to 14% to compensate the effect of polymer degradation. Further improvements in the process can be made by altering some other parameters and factors such as changing the polymer quenchant, addition of additives change in quench pressure etc.

REFERENCES: [1] ASM Handbook Volume 1 - Properties and Selection [2] Irons Steels and High Performance Alloys


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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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