Carburization: A study on the Case Hardening of Steels. By, Hans Cocks

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1 Abstract Carburization: A study on the Case Hardening of Steels By, Hans Cocks The objective of this report is to examine and identify the case depth of differently cooled but equivalently carburized specimens. The case depth was observed in two ways: qualitatively via changes in microstructure accomplished through optical microscopy; quantitatively via microhardness readings as a function of distance from the edge of the sample using the Vickers microhardness tester. For both the furnace cooled and water quenched samples the effective case depth was observed to be approximately 0.8mm. Using these results the carbon concentration at this depth was determined to be wt% C. Additional aims of this report are to gauge the effect of heat treatment on case depths, microstructures, and resulting mechanical properties. From the results obtained case depth appeared to be equivalent in both samples, with the furnace cooled sample being harder to identify. The microstructures in the quenched sample vs the furnace cooled sample differed greatly along the sample edges primarily by a lack of martensite(a diffusionless phase) in the furnace cooled sample which also led to its relative reduced hardness. Introduction The principles of action for case hardening rest with the concept of Diffusion. Diffusion is material transport by the movement of atoms, an empty adjacent site and sufficient energy for the movement of the atom must be satisfied; thus two things are required for this phenomenon to occur, time and elevated temperature [1]. When the temperature in a metal increases so does the thermal energy and vibration of atoms, bonds between atoms also become stretched and weaker, the atoms are more likely to move from their lattice sites and the number of lattice vacancies increases exponentially [2]. These effects lead to a redistribution of elements in a material, and can be utilized to introduce new elements into the material [2]. Diffusion has a wide array of applications in materials science including annealing, 1

2 heat treatment, age hardening of alloys, and oxidation and creep [2]. There are two classifications of diffusion, steady-state and non-steady state; Steady state diffusion is indicated by a diffusion rate independent of concentration, and is described by Fick s 1 st Law: J x =-D(dc/dx), where J x is the diffusion flux, D is the diffusion coefficient, and (dc/dx) is the concentration gradient which is the driving force for diffusion, remaining constant in this case [1]. Fick s 2 nd law is used for non-steady state diffusion: (dc/dt)=d(d 2 c/dx 2 ), there are a variety of solutions which come up for this law, each one being dependent on the systems boundary conditions [1]. Diffusion is a structure sensitive process because the diffusion coefficient, D, depends on the structure of the parents (sample) crystal lattice and increases with lattice irregularities (short-circuit diffusion mechanisms) i.e. large number of vacancies, grain boundaries and dislocations [1]. Surface hardening of steel is an example of non-steady state diffusion often referred to as case hardening. Case hardening is a process by which the outside of a metal is hardened while keeping the interior of the metal soft. This process is accomplished through diffusion of another species, typically Carbon or Nitrogen, into the sample at high temperatures for variable lengths of time. This process works best for metals with low hardenability i.e. low carbon steels; the most commonly practiced method is carburization where low carbon steel is placed into an environment containing carbonaceous compounds then brought to a critical temperature between 815 o C and 1038 o C for a certain amount of time allowing for diffusion of the surrounding carbon to occur into the low carbon steel [2]. The carburized steel will have a gradient of high carbon at the surface to low carbon at the core as well as residual compressive forces introduced during the process [3]. Often the samples are quenched after carburization to form the diffusionless phase martensite, the hardest steel microstructure, due to the new higher carbon surface layer having improved hardenability [2]. The carburized and quenched steel could then undergo tempering to obtain desirable properties. Many applications are created using low carbon steels because of the ease at which they can be manufactured/machined [3]; subsequent case hardening is used to enhance the surface hardness of the application as well as improve the applications fatigue life [3]. The focus of this study is to experimentally connect the laws of diffusion to case depth predictions and to 2

3 gauge the effects of heat treatment on case depth, microstructures, and resulting mechanical properties. Experimental To begin, 16 quarter inch sections were cut from a cylindrical 1018 steel rod indicating a carbon concentration of 0.18%, the oxidized layers on each sample were then grinded off to avoid carburization complications. Apart from two control specimens, the samples were divided into two batches; each batch was packed into its own sealed stainless steel environmental bag filled with WILCARBO, a carburization media comprised of charcoal, graphite, alkali carbonate, and carbonate, then was placed in a furnace at 925 o C along with the controls. After 1 hour, one batch was removed; the specimens inside the bag were taken out and immediately quenched in cold water along with 1 control sample. The specimens from the other batch were removed from their bag at this time and, remaining control included, were placed in a furnace set to 750 o C. After 15 minutes the furnace was turned off and the samples were left to furnace cool. Using the water cooled cut-off wheel, the specimens were cut longitudinally through their centers; some samples were then mounted in bakelite using the hot-mount equipment. After grinding and polishing the two halves, 1 half underwent nital etching and micrographs were subsequently obtained using a compound optical microscope to examine the microstructures and corresponding case depths. The other polished half was used to obtain microhardness information. This data was collected using the Vickers hardness test which utilizes a 4 sided square based diamond pyramid indenter and a 500g load to make indentations; with proper calibration the diagonals of separate indents made progressively from the edge of the sample to its center were able to be accurately measured as a function of distance from the edge. Results and Discussion The principles of carburization are based upon the fact that diffusion will occur into another species when temperatures and time allow for it. This concept is distant depended from the source of the diffusing species. The goal of carburizing is to obtain a high-carbon martensitic case with good wear and fatigue resistance superimposed on a tough, low carbon steel core [4]. 3

4 Figure 1 shows a graph of the hardness profile for data on a carburized and furnace cooled sample as well as a carburized and quenched sample. A complete set of data from the control samples was unavailable and thus will not make an appearance in this analysis. It should be noted here however that in carburizing, more carbon is diffused into the surface of the sample, thus in a direct comparison to control samples, the edges of the carburized specimens will in theory have a much higher hardness than the control samples. This statement is supported by additional data from other experiments where the hardness of carburized specimens had a clear increase over non-carburized samples closer to the edges of the specimens [4, pp. 298,313,440]. Looking back to figure 1, the first data points are approximately 200um from the edge, each additional data point is taken approximately 300um closer to the center of the sample. The separation between points was done to avoid affecting the data via lattice deformation caused by each indent. This separation may have produced an unclear separation between carburized and uncarburized zones but the effective case depth appears to be 0.8mm based on the data shown. Using Ficks 2 nd law its possible estimate the carbon concentration at the effective case depth: (C (x,t) -C o )/(C s -C o )=1-erf(x/2sqrt(Dt)) C o initial carbon concentration = 0.18%; C S surface carbon concentration = 100%; x distance from edge = m; t is the time of carburization = 3600s; D the diffusion coefficient for the indicated temperature = x m 2 /s, the erf portion of the problem is the error function which is an indefinite integral describing concentration profiles at various times [1]. an erf(z) table is used in conjunction with this formula to obtain accuracy. Upon plugging in the appropriate values the carbon concentration at 0.8mm appears to be wt% C which is a large increase from 0.18 wt% C. This estimation uses a coefficient of diffusion for carbon into gamma iron obtained in literature [3, pp731/32]. This data is supported by changes in the microstructures moving from the edge of each sample to their cores. Figure 2 shows the middle edge of the furnace cooled and carburized sample. The microstructure appears to contain alpha ferrite and a finer pearlite. Figure 3 is the same sample showing the core microstructure only in this image there is much more ferrite and the pearlite 4

5 is coarser here. The appearance of more pearlite in the outer layer supports the increased hardness in that region as a result of the carburization; its finer detail also indicates less diffusion was allowed to occur near the edge of the sample which also increases the hardness by introducing more grain boundaries. The quenched sample is much easier to determine a case depth from. The appearance of martensite allows one to easily see the declining carbon content towards the center of the sample. Figure 4 shows the middle edge of the quenched sample. The microstructure appears to contain a large portion of martensite, a testament to the hardness here, as well as some alpha ferrite grains. Figure 5 shows the interior of the quenched sample which contains alpha ferrite, fine pearlite, and some martensite. The appearance of less martensite and some finer pearlite indicates a small amount of diffusion was allowed to occur here, the alpha ferrite is also more prevalent indicating a lower carbon concentration. The easiest way to explain the separation of hardness between the two differently treated samples is via the appearance of martensite. Because diffusion is not favored in the quenched sample more martensite forms as well as finer pearlite lending to its increased hardness throughout the specimen. With diffusion being allowed to occur in the other sample, the hardness is reduced. Conclusion This report has looked into the process of carburizing, its effects, and how different subsequent cooling affects the cased portion of the samples. The relative hardness at different intervals of each specimen was observed through the Vickers hardness test as well as through comparisons of the microstructure for each differently treated sample. The effective case depth for both samples was observed to be 0.8mm. The carbon concentration at this depth was then estimated to be wt% C. This concentration is not indicative of the hardness of each sample however as the different rates of cooling in each sample allowed for more and less diffusion to occur. In the quenched sample the hardness was observed to be much higher and this is a result of the formation of martensite. The slow cooled sample showed similar carbon contents at each interval only with more diffusion occurring giving a less hard profile. To increase the effectiveness of carburization it is best to quench a sample thereafter. 5

6 Vickers Hardness Value References [1] D.A. Porter, K.E. Easterling, M.Y. Sherif, Phase Transformations in Metals and Alloys, 3 rd Ed, CRC Press, [2] Wo, Amy, class ppts., Case Hardening, a case study for diffusion, WSU, Fall [3] W. D. Callister, D.G. Rethwisch, Materials Science and Engineering: An Introduction, eighth Ed., Wiley, [4] G. Krauss, Steels: Processing, Structure, and Performance, ASM International, Figures Figure Hardness Profile of samples Carburized and Quenched Carburized and Furnace Cooled 20 0 Graph describing the relative hardness of each differently treated sample as a function of distance from the edge of the sample. The sample data begins 200um from the edge and each reading is taken approximately 300um closer to the center of the sample. 6

7 Figure 2 Middle edge of the furnace cooled and carburized sample at 1,000X magnification. Microstructure appears to contain alpha ferrite and pearlite. Figure 3 Center of the furnace cooled and carburized sample at 1,000X magnification. Microstructure appears to contain alpha ferrite and some coarser pearlite. 7

8 Figure 4 Middle edge of the quenched and carburized sample at 1,000X magnification. Microstructure appears to contain alpha ferrite and martensite. Figure 5 Center of the quenched and carburized sample at 1,000X magnification. Microstructure appears to contain alpha ferrite, finer pearlite, and some martensite. 8