Fatigue Properties of a Microalloyed Steel as Hot Rolled Bar and in the Forged Condition

In an earlier post, it was shown that the fatigue properties of a component as-hot formed from a microalloyed steel compared favorably with the properties of quenched and tempered medium carbon steels.  Comparable fatigue properties were achieved provided hardness values were approximately the same.  This meant that the microalloyed steel could replace a quenched and tempered steel of similar hardness, thereby eliminating the need for heat treatment.

More recently, interest developed in comparing the fatigue properties of large diameter as-hot rolled microalloyed steel bars with those of large as-forged components obtained from the same steel composition.  For this study, a vanadium-containing microalloyed steel, 1538MV, was used.  Hot rolled steel bars, 105mm in diameter, were obtained for determining as-hot rolled bar properties.  For examining the properties of forgings, several commercially produced automotive crankshafts, forged from 120mm diameter bars, were also secured.  The chemical composition of the hot rolled bars, along with a typical composition for the forgings, is shown below in Table 1.

Post 25 - Table 1.1

The mechanical properties and hardness of the hot rolled bars were obtained along with those of the forged crankshafts.  The properties of the forged crankshafts were obtained from specimens machined from the counter weights of the crankshafts.  Average properties are shown in Table 2.  Efforts were made to monitor the production of the as-hot rolled bars in order to assure properties close to those of the crankshafts.  As can be seen, similar strength and hardness values were obtained. The hot rolled bars, however, did exhibit slightly higher strength and hardness values.  The microstructures of both the hot rolled bars and the crankshafts were ferrite and pearlite.

Post 25 - Table 2

Fatigue properties were obtained for both the hot rolled bars and the crankshafts.  In the case of the crankshafts, the specimens were again obtained from the counter weight sections.  The comparative strain-life fatigue properties are shown in Figure 1.  The hot rolled bar properties are identified as Iteration No. 131, and the as-forged crankshaft properties as Iteration No. 133.  The crankshaft data, compared to the hot rolled bar data, shows somewhat inferior performance at long life, but higher performance at short life.  The long life difference could be attributed to the differences in strength and hardness between the hot rolled bars and the forged crankshafts.

No. 25 Figure 1
Figure 1

The fatigue properties of the crankshafts were obtained from three different locations in the counterweights. The aim was to determine if any variation in fatigue performance occurred due variations in deformation resulting from the forging process.  The fatigue data for the crankshafts, shown in Figure 1, were extracted and re-plotted in Figure 2.  The differing locations are identified arbitrarily as B, C and D.  As can be seen, no difference in fatigue properties can be observed.

No. 25 Figure 2.jpg
Figure 2

The results of this study suggest that the fatigue properties of microalloyed steel components, forged  from hot rolled microalloyed steel bars, can be estimated from the fatigue properties of the bars provided that the strength and hardness values of the two product forms are comparable.

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Effect of Steel Composition on the Case and Core Fatigue Properties of Low Alloy Carburized Steels

In earlier posts, limited data indicated that the fatigue properties of the cores of carburized components do not vary significantly with the composition of low alloy steels.  There was evidence, however, that the fatigue properties of the cases of carburized components are influenced by steel composition.  Better case fatigue performance was exhibited by more highly alloyed steels.

To more completely examine this issue, case and core fatigue data were analyzed for three low alloy carburizing steels.  The three steel grades were SAE 4320, SAE 9310 and 20MnCr5.  SAE 4320 is a Ni-Cr-Mo steel, SAE 9310 is a high-nickel (3%) steel, and 20MnCr5 is a Mn-Cr low alloy steel.   Carburized cores were simulated by heat treating steel bars using a carburizing thermal cycle without the presence of carbon in the furnace atmosphere.  Carburized cases were simulated by through-carburizing fatigue specimens using a carburizing thermal cycle in a carbon-bearing furnace atmosphere.

The mechanical properties for the case and core simulations for the three steel grades are shown below.

Table Blog Post 24

Some inconsistencies were observed in the tensile properties in the case simulations due to low ductility and early failure.  The microstructures in the case simulations were martensite, and in the core simulations mixtures of martensite, bainite and ferrite were observed.

Since, in recent posts, it was noted that core fatigue properties varied with hardness resulting from changes in cooling rate following carburizing, core data for this analysis was obtained at constant hardness.

The strain controlled fatigue properties for the simulated carburized cores for all three steels are shown in Figure 1.  The data for SAE 4320 is given by Iteration No. 122, for SAE 9310 by Iteration No. 125 and for 20MnCr5 by Iteration No. 128.  It can be seen that the fatigue properties for the three steel grades all fall within a very narrow band.  Thus, at constant hardness, equivalent fatigue properties were observed for the carburized cores of all three steels.

No. 24 Fig. 1
Figure 1

Figure 2 shows the strain controlled fatigue properties for the simulated carburized cases for all three steels.  The data for SAE 4320 is given by Iteration No. 167, for SAE 9310 by Iteration No. 168 and for 20MnCr5 by Iteration No. 170. As can be seen the more highly alloyed SAE 9310 and SAE 4320 steels exhibit better fatigue performance than the 20MnCr5 steel.

No. 24 Fig. 2
Figure 2

Thus the data indicates that, while equivalent core fatigue properties can be obtained for carburized steels at various alloy levels and at constant hardness, applications requiring superior case performance require more highly alloyed steel grades.

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Additional Data on the Effect of Cooling Rate on the Fatigue Properties of Carburized Steels

In the previous post, the effects of a cooling rate change after carburizing, due to a change in steel bar diameter, on core fatigue properties was examined.  The data was developed for a Ni-Cr-Mo low alloy steel (SAE 4320), and was aimed at demonstrating the effects of varying section size on the core fatigue properties of carburized components.  It was shown that smaller bar diameters resulted in higher percentages of martensite, higher hardness and improved fatigue properties compared to larger bar diameters.

Additional data have been developed for a Mn-Cr low alloy steel, 20MnCr5.  Two bar diameters were again evaluated: 15.2 mm (0.6 in.) and 60.9 mm (2.4 in).  Both bar diameters were heat treated using a carburizing thermal cycle, again without the presence of carbon in the furnace atmosphere.  As in the previous post, the aim was to simulate carburized cores at different cooling rates.  The mechanical properties and hardness values obtained for the two bar diameters are shown in the table below.

Table 23

The 15.2 mm diameter bar developed a microstructure estimated at 90% martensite, 5% bainite and 5% ferrite.  The 60.9 mm diameter bar exhibited a microstructure estimated at 50% martensite, 10% bainite and 40% ferrite.

Comparative strain-controlled fatigue properties are shown in Figure 1.  The 15.2 mm diameter bar is represented by Iteration No. 130, and the 60.9 mm diameter bar by Iteration No. 128.

It can be seen that the smaller diameter, higher hardness bar exhibits improved fatigue properties compared to the larger diameter bar, especially as run-out is approached at lower strain amplitudes.

These results correlate with the earlier results in the previous post, and again indicate that variations in the section size of a carburized component can affect cooling rate after carburizing.  This in turn can result in changes in core fatigue properties.

No. 23 Fig. 1
Figure 1

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Effect of Cooling Rate after Carburizing on the Fatigue Properties of Carburized Steels

In earlier posts, the fatigue properties of various carburized low alloy steels were examined.  Both case and core properties were evaluated.  Since carburized components are manufactured in various section sizes, it can be expected that cooling rates in the cores following carburizing will also vary, and have a subsequent effect on core microstructure and properties.

A study was initiated which involved subjecting low alloy steel bars of various diameters to a carburizing thermal cycle without the presence of carbon in the furnace atmosphere.  The goal was to simulate carburized cores with varying cooling rates by varying section size.   Microstructures, properties and fatigue performance were then   determined.

Initial data was developed on SAE 4320 steel, a nickel-chromium-molybdenum low alloy steel.  Two bar diameters were employed: 15.2 mm (0.6 in.) and 30.5 mm (1.2 in.).  The mechanical properties and hardness values of the two bar diameters following the carburizing thermal cycle are shown below.

Table blog post 22

The 15.2 mm diameter bar developed a microstructure which was estimated at 90% martensite, 8% bainite and 2% ferrite.  The microstructure of the 30.5 mm diameter bar was estimated at 30% martensite, 60% bainite and 10% ferrite.

The strain controlled fatigue curves for both bar diameters are shown in Figure 1.  Iteration 122 shows the fatigue properties for the 30.5 mm diameter bar, and Iteration 124 shows the fatigue properties for the 15.2 mm diameter bar.

It can be seen that the smaller diameter bar, with the higher percentage of martensite and higher hardness, exhibits improved fatigue performance compared to the larger diameter bar.

This data indicates that, in applications involving heavier section components, where the fatigue properties of the cores of carburized components may be critical, attention needs to be given to the cooling conditions following carburizing.

No. 22 Fig. 1
Figure1

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Additional Effects of Sulfur on the Fatigue Properties of Low Alloy Steels

In the previous posting, it was shown that, as a result of the presence of elongated manganese sulfide inclusions, the fatigue properties of low alloy steels can exhibit directionally dependent behavior.  Fatigue properties obtained in a test direction transverse to the elongated manganese sulfides were found to be inferior to those obtained in a test direction parallel to the elongated manganese sulfides.  In this posting, fatigue properties in the transverse direction are explored further as functions of hardness and sulfur level.

SAE 4140 steel was again selected for evaluation.  Heats of steel with three different sulfur concentrations were examined, and specimens from each were evaluated at two hardness levels.  As described in the previous posting, transverse testing was conducted on specially prepared, quenched and tempered, steel sections which permitted machining fatigue specimens transverse to the elongated manganese sulfides.

The table below summarizes the pertinent details of composition and hardness for the steels tested.

Blog 21 - Table

The microstructure of all of the test sections was martensite.

Figure 1 shows the strain-life fatigue curves obtained for the three sulfur concentrations at the lower hardness level shown in the table above.  The strain-life curve for 0.004 wt% sulfur is given by Iteration No. 99, the strain-life curve for 0.012 wt% sulfur is given by Iteration No. 80, and the strain-life curve for 0.077 wt. % sulfur is given by Iteration No. 81.  As can be seen, fatigue performance deteriorates as sulfur level is increased, especially in the high strain amplitude, short life regime.  The decrease in performance on increasing sulfur from 0.004 wt. % to 0.012 wt. % is relatively modest; however, the reduction in performance on increasing the sulfur concentration to 0.077 wt. % is much more significant.

No. 21 Figure 1Figure 1

Figure 2 gives the strain-life curves developed for the three sulfur concentrations at the higher hardness level. Iteration No. 98 gives the strain-life curve for 0.004 wt.% sulfur, Iteration No. 76 gives the strain-life curve for 0.012 wt.% sulfur, and Iteration No. 77 shows the strain-life curve for 0.077 wt.% sulfur.  As in the case of the lower hardness level, fatigue performance is reduced as sulfur level is increased.  In this case, however, the decrease in performance on increasing sulfur level from 0.004 wt. % to 0.012 wt. % is more pronounced in the high strain amplitude, short life regime.

These results reinforce the data exhibited in the last posting.  Fatigue performance is dependent on sulfur level when the loading direction is transverse to the elongated manganese sulfide inclusions.  Also there appears to be a somewhat higher sensitivity to sulfur level as hardness is increased.  These results need to be considered in component design.

No. 21 Figure 2Figure 2

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The Effects of Sulfur on the Fatigue Properties of Low Alloy Steels

It is well known that sulfur has a major impact on some of the properties of low alloy steels.  Sulfur is present in the form of manganese sulfides, which are highly plastic at hot working temperatures.  As a result, during the hot rolling of various steel mill products, these sulfides become elongated in the hot rolling direction.  This in turn imparts directionality to some of the properties.  For example, the values of ductility and notch toughness, when measured transverse to the elongated manganese sulfides, are often significantly less than those obtained from measurements longitudinal or parallel to the elongated sulfides.  It is of interest therefore to determine if strain controlled fatigue properties exhibit the same directionality.

SAE 4140 steel, quenched and tempered to a constant hardness, was selected for evaluation.  Heats of steel with two different sulfur concentrations were examined.  Testing in the longitudinal direction was carried out on through-induction hardened steel bars.  Transverse testing was conducted on specially prepared, quenched and tempered, steel sections which permitted machining fatigue specimens transverse to the elongated manganese sulfides.

The table below summarizes the pertinent details of composition, test orientation and properties for the steels tested.

chart - post 20
The microstructure of the through-induction hardened bars used for longitudinal testing was a mixture of martensite and a small amount of bainite.  The microstructure of the specially prepared, quenched and tempered, sections used for transverse testing was martensite.

As can be seen, the ductility in transverse direction was significantly less than that observed in the longitudinal direction for both sulfur levels.  Furthermore, there was no effect of sulfur level on ductility in the longitudinal direction; however, there was a significant reduction in ductility in the transverse direction when sulfur level was increased.

Figure 1 shows the strain-life fatigue curves obtained in both the longitudinal and transverse directions for the two low sulfur levels shown in the table above.  The strain-life curve for the longitudinal direction is given by Iteration No. 96, and the strain-live curve for the transverse direction by Iteration No. 80. The fatigue performance in the longitudinal direction is slightly better than in the transverse direction especially in the high strain amplitude, short life regime.

No. 20 Figure 1Figure 1

Figure 2 gives the strain-life curves developed in both orientations for the high sulfur levels given in the table above. Iteration No. 97 gives the strain-life curve in the longitudinal direction, and Iteration No. 81 shows the strain-life curve in the transverse direction. The fatigue properties in the longitudinal direction are clearly superior to those in the transverse direction at all levels of strain amplitude.

The strain-life curves in the longitudinal direction appear to show little dependence on sulfur level, whereas there is a stronger dependence of fatigue performance on sulfur level in the transverse direction.  This finding parallels the effects of sulfur on ductility as shown in the table above.  Thus, to the extent that manganese sulfides are present in a component subject of fatigue loading, the direction of loading needs to be considered in design.

In future postings, the effects of sulfur will be investigated further, not only as a function of sulfur level, but also as a function of hardness.

No. 20 Figure 2Figure 2

 

 

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Additional Comparison of Case Hardened Properties for Alloy Steels

 

In the last recent posting, the case and core properties of carburized SAE 86B20 and SAE 4620 were compared.  The data showed both grades to have comparable core fatigue properties; however the more highly alloyed SAE 4620 exhibited superior case fatigue properties. In this posting, the effect of alloy content on the case and core fatigue properties of alloy steels is explored further with the aim of confirming the earlier findings.

As has been noted in a number of earlier postings, carburizing is usually employed on carbon or low alloy steels containing approximately 0.2-0.25 wt. % carbon.  Carbon is diffused into the surface of a part during heat treatment resulting in a high carbon (0.8 wt. %), high hardness case and a lower carbon, softer core.  Comparisons of case and core properties for various alloy steels are important in facilitating the selection of a particular steel grade for a given application.

In this posting, the fatigue properties of carburized SAE 86B20 are compared with carburized SAE 4320.  As was pointed out in the last posting, SAE 86B20 is a boron-enhanced variant of SAE 8620, which is a nickel-chromium-molybdenum steel.  SAE 4320 is also a nickel-molybdenum grade, but contains a significantly higher amount of nickel (1.65-2.00% by weight).  As in the earlier comparison, the case properties for each grade were developed through simulation by diffusing carbon completely through fatigue specimen blanks. The properties of the core were simulated by subjecting specimens to the carburizing thermal cycle absent the presence of carbon in the atmosphere.

The table below summarizes the mechanical properties and hardness values obtained for the two steel grades.

chart

The tensile strength for SAE 86B20 at the case location was lower than expected due to a very low ductility; this resulted in failure during tensile testing before reaching the expected ultimate tensile strength.  For the same reason, a reliable value of ultimate tensile strength could not be obtained for SAE 4320 at the case location.

The microstructures obtained for SAE 4320 consisted of martensite, bainite and ferrite in the core, and martensite in the case.  For SAE 86B20, the martensite was observed at both the case and core locations.

Figure 1 shows the strain-life fatigue curves obtained for the core location of each of the two steel grades.  The strain-life curve for SAE 86B20 is given by Iteration No. 74, and the strain-life curve for SAE 4320 is given by Iteration No. 49.  The data and the calculated strain life curves show comparable fatigue performance for both steel grades.

No. 19 Figure 1
Figure 1

Figure 2 gives the fatigue properties of the case locations for the two steel grades.  Iteration No. 75 gives the strain-life curve for SAE 86B20, and Iteration No. 50 shows the strain-life curve for SAE 4320. In this location, the fatigue properties of SAE 4320 are significantly better than those of SAE 86B20.

These results are very similar to those reported in the earlier posting where SAE 4620 was compared with SAE 86B20.  In that case, as well as for the data shown here, the more highly alloyed steel grade exhibited superior case fatigue performance.  Thus in demanding applications, where case properties are critical, more highly alloyed steel grades would be preferred.

No. 19 Figure 2
Figure 2

 

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Comparison of Case Hardened Properties for Two Different Alloy Steels

In the most recent posting, the case and core properties of case hardened steels were compared for carburized and surface induction hardened steel grades.  In this posting, the case and core fatigue properties for two different carburized alloy steel grades are compared.

As was noted earlier, carburizing is usually employed on carbon or low alloy steels containing approximately 0.2-0.25 wt. % carbon.  Carbon is diffused into the surface of a part during heat treatment resulting in a high carbon (0.8 wt. %), high hardness case and a lower carbon, softer core.  Comparisons of case and core properties for various alloy steels are of interest in terms of helping in the selection of a particular steel grade for a given application.

In this posting, the fatigue properties of carburized SAE 86B20 are compared with carburized SAE 4620.  SAE 86B20 is a boron-enhanced variant of SAE 8620, which is a nickel-chromium-molybdenum steel.  SAE 4620 is a nickel-molybdenum grade containing a high amount of nickel (1.7% by weight).  For each grade, the properties of the case were developed through simulation by diffusing carbon completely through fatigue specimen blanks. The properties of the core were simulated by subjecting specimens to the carburizing thermal cycle absent the presence of carbon in the atmosphere.

The table below summarizes the mechanical properties and hardness values obtained for the two steel grades.

18

The lower than expected tensile strength for 86B20 at the case location was due to a very low ductility; this resulted in failure during tensile testing before reaching the expected ultimate tensile strength. The microstructures obtained for SAE 4620 were a mix of martensite, bainite and ferrite in the core, and martensite in the case.  For SAE 86B20, the microstructure at both the case and core locations was martensite.

Figure 1 shows the strain-life fatigue curves obtained for the core location of each steel grade.  The strain-life curve for SAE 86B20 is given by Iteration No. 74, and the strain-life curve for SAE 4620 is given by Iteration No. 47.  The calculated strain life curves show somewhat better fatigue performance for the SAE 86B20 at long life.  The data points however show approximately comparable behavior.  It should be noted that the core hardness of SAE 86B20 exceeded that of SAE 4620.

No. 18 Figure 1Figure 1

Figure 2 gives the fatigue properties of the case locations for each steel grade.  Iteration No. 75 gives the strain-life curve for SAE 86B20, and Iteration No. 48 shows the strain-life curve for SAE 4620.

In this location, the fatigue properties of SAE 4620 are clearly superior to those of the SAE 86B20.  This suggests that, under conditions of demanding fatigue service, SAE 4620 should be selected over SAE 86B20.  This would be especially true where superior case properties are required.

No. 18 Figure 2Figure 2

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Comparison of Case Hardening by Carburizing Versus Surface Induction Hardening

In past postings, the case and core properties of case hardened steels have been discussed.  These discussions have included the fatigue properties of both carburized and surface induction hardened steels.

Carburizing is usually employed on carbon or low alloy steels containing approximately 0.2-0.25 wt. % carbon.  Carbon is diffused into the surface of a part during heat treatment resulting in a high carbon (0.8 wt. %), high hardness case and a lower carbon, softer core.  Surface induction hardening is utilized on higher carbon steels (~0.7 wt. %).  During heat treatment, the surface of a part is rapidly heated by induction and then cooled to create a high hardness case coupled with a lower hardness core.  Of interest are direct comparisons of the fatigue properties that can be obtained with either case hardening method, since such a comparison would help in the selection of a given combination of steel grade and processing for a particular application.

In this posting, the fatigue properties of carburized SAE 4620 are compared with surface induction hardened SAE 1070.  For the SAE 4620 the properties of the case were developed through simulation by diffusing carbon completely through fatigue specimen blanks. The properties of the core were simulated by subjecting specimens to the carburizing thermal cycle absent the presence of carbon in the atmosphere.  For the SAE 1070 the properties of as-hot rolled bar stock were used to simulate the core, and the case was simulated by through induction hardening companion bar stock.

The table below summarizes the mechanical properties and hardness values obtained for the two steel grades and processes.

17 - chart

It should be noted that equivalent hardness values were observed for the core location of each grade as well as for the case location.  The microstructures obtained for SAE 4620 were a mix of martensite, bainite and ferrite in the core, and martensite in the case.  For SAE 1070, since as-hot rolled bar was used to simulate the core, the microstructure was pearlite with some ferrite.  The case, simulated by through induction hardening, was a mix of bainite, pearlite and a small amount of martensite.

Figure 1 shows the strain-life fatigue curves obtained for the core of each steel grade.  The strain-life curve for the simulated core of surface induction hardened SAE 1070 is given by Iteration No. 36, and the strain-life curve for the simulated core of carburized SAE 4620 is given by Iteration No. 47.  As can be seen, the fatigue properties for both test iterations are the same.

No. 17 Figure 1 compressedFigure 1

Figure 2 gives the fatigue properties of the simulated cases for both surface induction hardened SAE 1070 and carburized SAE 4620.  Iteration No. 37 gives the strain-life curve for surface induction hardened SAE1070, and Iteration No. 48 shows the strain-life curve for carburized SAE 4620.

No. 17 Figure 2 compressedFigure 2

In this instance, the fatigue properties of carburized SAE 4620 are superior to those of the surface induction hardened SAE 1070.  This suggests that, where case fatigue properties are an important consideration, the carburized low alloy SAE 4620 should be selected as opposed to the surface induction hardened SAE 1070.

Future postings will provide further comparisons of these two case hardening methods, and will also examine the effects of alloy content on the fatigue properties of case hardened steels.

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Comparison of Through-Induction Hardening with Conventional Hardening

Two methods are often used to through harden and low alloy steels.  The well recognized conventional hardening method involves heating to an appropriate austenitizing temperature for a specific length of time in a furnace with or without a protective atmosphere. This is followed by quenching in oil or water, and then tempering at a specific sub-critical temperature in order to achieve desired mechanical properties and hardness.  A second method involves rapid heating to the austenitizing temperature in an induction coil for a length of time sufficient to obtain a uniform temperature followed by rapid cooling.  The mechanical properties and hardness are usually dictated by the cooling conditions following austenitizing, since sub-critical tempering is not generally used.  While comparable mechanical properties and hardness can often be achieved with both methods, microstructures can vary significantly.

Of interest is a comparison of the fatigue properties that can be obtained with either method.  The AISI Bar Steel Fatigue Database contains data for SAE 4140 low alloy steel which was through hardened using both conventional and through-induction hardening.  While the data was developed for bar stock as opposed to fabricated parts, it does provide a means of comparing the fatigue properties for the two heat treating methods.

The table below summarizes the mechanical properties and hardness values obtained for the two processes.

post 16 table

Somewhat higher values of yield strength and tensile strength were achieved with the induction hardened process.  The microstructure obtained after conventional quenching and tempering was tempered martensite, whereas the microstructure observed in the induction hardened condition contained a large amount of bainite and a lesser amount of ferrite.

Figure 1 shows the strain-life fatigue curves obtained for both heat treating conditions.  The strain-live curve for conventional quenching and tempering is given by Iteration No. 68, and the strain-life curve for induction hardening is given by Iteration No. 93.


No. 16 Figure 1
Figure 1

It can be seen that comparable fatigue properties are obtained for both hardening processes.  The fitted strain-life curves show a slight advantage for through-induction hardening at fatigue lives from 103-105 reversals and somewhat better performance for conventional hardening at lives greater than 106 reversals.

The decision as to which process to use can be based on manufacturing considerations and other property concerns, e.g. notch toughness, rather than on fatigue requirements.  It should be noted that, while parts of almost any configuration can be hardened using the conventional process, the through-induction hardening process is constrained by part geometry.  Induction coils cannot be designed for hardening many complex parts.

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