Strength and Fatigue Life versus Carbon Content at High Hardness (60 HRC)

The strength, fatigue and hardness data for several iterations with a high hardness level, around 60 HRC are shown in Table 1. The steels in these iterations are both plain carbon and low alloy grades with carbon contents ranging from 0.50% to 0.95%. The heat treatments used to obtain this hardness level were quench and tempering, carburizing and induction hardening. All of the samples during this time period were finish ground and polished after heat treatment, with the exception of iteration 41 which was not ground to preserve the intergranular oxidation (IGO) present at the surface. All of the samples were through hardened in the gage length.

Table 1
Blog 33 Table 1

A graphical representation of the data is shown in Figure 1. The graph shows the yield strength and ultimate strength decrease as the steel carbon content increases. Per SAE J413 the expected tensile strength at 60HRC is 2469 MPa. This is in good agreement with the SAE 6150 and SAE 9254 quench and tempered steels at the left end of the curve, which are 2343 MPa and 2450 MPa respectively. However, as the carbon content increases to 0.95% at the right end of the curve the ultimate strength decreases to about 1000-1500 MPa. All of the data follows this trend with the exception of the SAE 1552 induction hardened steel (iteration 92) which has the lowest strength at 933 MPa. An examination of the microstructure revealed the probable cause. The typical grain diameter of iteration 92 is 76 microns which is significantly larger than the other iterations at approximately 13 microns. In addition to the steel carbon content affecting the strength at this high hardness level it appears the grain size can also have a significant effect. Iteration 92 is approximately an ASTM E112 grain size 4.5 while the other samples are around grain size 10. A grain size of 4.5 is not unusually large for an induction hardened component, however a grain size 10 is unusually small. While the carbon content appears to have a significant effect on strength it does not appear to affect the fatigue life. The fatigue strength for one million cycles was relatively constant for all of the samples at approximately 500 MPa.

Blog 33 Figure 1Figure 1

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Carburized Core Properties When Hardness is Controlled by Cooling Rate

In the previous article we examined the relationship between the core hardness of carburized steels to both strength and fatigue. In that study the hardness was controlled by the tempering temperature used during the quench and temper heat treatment. In actual practice the core hardness of carburized parts is a function of the steel hardenability, the cooling rate of the quenchant used, and the cross section size of the part. Hence, with a given steel and quenchant the hardness is controlled by the cooling rate.

In iterations 119-130 test bars were produced from 8620, 4320, 9310 and 20MnCr5 steel. These bars were intended to represent the core of carburized components at three different hardness levels. Rather than to control the hardness by quench and tempering the hardness was controlled by machining bars to various diameters and then oil quenching and tempering at 177 C. This more closely represents what occurs during actual carburizing.

Table 1 shows the fatigue strength, mechanical property and hardness data for iterations 119-130. Again, the fatigue strength is the stress level to achieve 1 million cycles.

Blog 32 - Table 1Table 1: Core Hardness Properties Iterations 119-130

Figure 1 shows the ultimate strength versus hardness for the four grades of steel. Shown are the actual data points as well as the upper and lower bounds. A nearly linear relationship is shown between hardness and strength. Also shown are the upper and lower bounds from the previous article where the relationship did not appear to be linear. There is overlap in the two data sets, however the most recent data appears to be in the lower portion of the first. Once again, the steel grade or alloy content does not appear to be significant. The high alloy 9310 steel and medium alloy 4320 steel do not appear to be any stronger than the low alloy 8620 and 20MnCr5 steels at any given hardness level.

Blog 32 - Fig 1Figure 1: Ultimate Strength versus Core Hardness, Iterations 119-130

Figure 2 shows the fatigue strength versus hardness. Again, the actual data points are shown along with the upper and lower bounds. The relationship between fatigue strength and hardness is linear and the slope appears to be similar to the strength versus hardness plot above. Also shown are the upper and lower bounds from the prior article. In both cases the relationship is linear, however in the previous data set the slope is more shallow and there appears to be more scatter or variation.

Blog 32 - Fig 2
Figure 2: Fatigue Strength at 1 Million Cycles versus Core Hardness, Iterations 119-130

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Carburized Steel Core Strength and Fatigue Life as a Function of Hardness

In this article we will examine the strength and fatigue life found in the core of a part when using typical low carbon alloy carburizing steels. Table 1 shows the data found with this type of material and heat treatment through Iteration No. 110. The data shown are the iteration number, steel grade, fatigue strength at one million cycles, yield strength, ultimate strength, reduction of area, elongation and reported hardness. These samples were all quench and tempered using simulated carburizing cycles. However, in many cases the tempering temperature was elevated in order to control the level of hardness to a more typical value found in a part. Because of the small test-bar diameter, the core hardness of all grades would be quite high if the normal 177 C tempering temperature was used after oil quenching.

Blog 31 Table 1

Figure 1 shows ultimate tensile strength versus hardness. It is evident there is a reasonably good relationship between hardness and strength for these samples. Higher hardness provides increased strength within the range shown. Most of the grades in this study were the more common low-alloy variety such as 8620, 5120, 8822, 41B17M and 86B20. However, a few iterations used medium and high-alloy grades such as 4620, 4320 and 9310, and these are pointed out in the figure. It is interesting to note these grades follow the same curve of strength versus hardness and do not offer any benefit in this regard.

Blog 31 Fig 1
Figure 1: Ultimate strength versus hardness for low carbon alloy
carburizing steels through iteration 110

Figure 2 shows the fatigue strength at one million cycles versus hardness. There appears to be a relationship between high-cycle fatigue strength and hardness, but it is not as well defined as the previous example. Fatigue strength appears to increase with increasing hardness, but there is a significant amount of scatter. This indicates there may be other factors affecting the life other than hardness, such as the heat of steel, the heat treatment, and the machining process. Once again, the higher alloy steels do not appear to offer any benefit as all data fall within the same scatter band.

Blog 31 Fig 2
Figure 2: Fatigue strength versus hardness for low carbon alloy
carburizing steels found through iteration 110.

It must be remembered we are only looking at the core properties in this comparison. An actual carburized component is a composite of the case and core. The performance of only the case or core by itself may not be indicative of the performance of the final part.

 

 

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Additional Data on the Effects of Cooling Rate on the Overload Fatigue Properties of Carburized Steels

In the previous post (Dec. 30, 2016), the effects of cooling rate on both constant amplitude and overload core fatigue properties of a carburized Ni-Cr-Mo alloy steel (4320) were compared.  As was noted, the study involved subjecting 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 sizes.  Test results showed constant-amplitude fatigue performance improved with the development of higher hardness at higher cooling rates.  Results also indicated fatigue performance under overload conditions improved.

A similar comparison has been completed on a Mn-Cr low alloy carburized steel, 20MnCr5.  Two bar diameters were evaluated as before: 15.2 mm (0.6 in.) and 60.9 mm (2.4 in).  Both bar diameters were heat treated using a carburizing thermal cycle, 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-blog-30

The constant amplitude strain-controlled fatigue results for the two bar diameters, described in an earlier post, are shown in Figure 1.  The 60.9 mm diameter bar is represented by Iteration No. 128, and the 15.2 mm diameter bar is represented by Iteration No. 130.  The higher hardness of the smaller diameter bar, resulting from a higher cooling rate, shows better fatigue performance particularly in the runout regime.

no-30-fig-1Figure 1

To examine the effects of overloads during fatigue testing, a test protocol was used as in previous posts (Dec. 30, 2016) and illustrated in Figure 2.  The load history in the protocol consists, which has been described in other posts, was undertaken, and which is shown in Figure 2.  The load history in the protocol consists of repeated blocks, each consisting of one fully reversed overload cycle and a series of small cycles with the same maximum strain as the overload cycle.  Effective strain-life data is determined for the small cycles, which can then be compared to results obtained under fully reversed constant-amplitude conditions.

no-30-fig-2
Figure 2

In Figure 3, Iteration No. 152 gives the overload fatigue results for the 60.9 mm diameter bar, and Iteration No. 154 shows the results for the 15.2 mm diameter bar.  As in the case of the 4320 overload data described in the previous post, the data scatter band for the 15.2 mm diameter bar lies slightly above that of the 60.9 mm diameter bar.  This indicates improved overload fatigue performance results from higher hardness developed through higher cooling rates.

no-30-fig-3-updated
Figure 3

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Effect of Cooling Rate on the Overload Fatigue Performance of Carburized Steels

In earlier posts, the effect of cooling rates on the core fatigue properties of carburized steels were examined.  The study 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 sizes.

An initial series of 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-29The constant amplitude, strain controlled fatigue properties 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 hardness, exhibits improved fatigue performance compared to the larger diameter bar.

In follow-up testing, the two bar diameters of SAE 4320 were subjected to overload fatigue testing to determine if hardness influenced overload fatigue performance.  As was described in other posts, to simulate the effects of overloads on fatigue performance, a fatigue testing protocol was implemented in which high amplitude cycles are inserted between groups of low amplitude cycles.  The test protocol is shown schematically in Figure 2.  As can be seen, the load history consists of repeated blocks, each consisting of one fully reversed overload cycle and a series of small cycles with the same maximum strain as the overload cycle.  Effective strain-life data is determined for the small cycles, which can then be compared to results obtained under fully reversed constant amplitude conditions.

no-29-fig-1Figure 1

no-29-fig-2Figure 2

Figure 3 shows a simple plot of the overload fatigue data obtained for the two SAE 4320 bar diameters.  Iteration 146 gives the results obtained for the 30.5 mm diameter bar, and Iteration 148 shows the results obtained for the 15.2 mm diameter bar.  The data exhibits some scatter, but the band for the 15.2 mm diameter bar lies at the upper edge and above the band for the 30.5 mm diameter bar.  This indicates that the higher hardness of the 15.2 mm diameter bar resulted in improved overload fatigue performance.

It should be noted however that, due to the scatter, additional results are needed to more completely define the role of hardness on overload fatigue.

no-29-fig-3Figure 3

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Effects of Sulfur and Overloads on the Fatigue Performance of Low-Alloy Steels

In earlier posts, the effect of sulfur on the fatigue performance of low-alloy steel was examined.  It was noted that sulfur is present as manganese sulfides in steels, and these sulfides are highly plastic at hot working temperatures.  During hot working the manganese sulfides elongate in the hot working direction giving rise to directionality effects for some mechanical properties.  When testing is carried out transverse to the elongated manganese sulfides, properties such as ductility and notch toughness are significantly reduced compared to results obtained parallel to the manganese sulfides.  The earlier posts also showed constant amplitude fatigue properties exhibited a directional behavior.  Results obtained from testing transverse to elongated sulfides showed fatigue properties were again significantly reduced compared to the fatigue properties observed from testing parallel to elongated sulfides.  The degree of reduction in fatigue properties increased with increasing sulfur level and hardness.

In a more recent study, the effects of overloads on the fatigue properties of a low-alloy steel were examined.  Heats of SAE 4140 steel with three different sulfur levels were secured.  Testing was carried out on specially prepared quenched and tempered steel sections which permitted machining fatigue specimens transverse to elongated sulfides.  Test identifications and associated sulfur levels are shown in Table 1.  The steel sections were all heat treated to HRC 40.

blog-post-28-table-1

Similar to what was noted in the most recent post, Effect of Overloads on the Fatigue Performance of a Forged Microalloyed Steel, a fatigue testing protocol was implemented in which high-amplitude cycles are inserted between groups of low-amplitude cycles.  The test protocol is shown schematically in Figure 1.  As can be seen, the load history consists of repeated blocks, each consisting of one fully reversed overload cycle and a series of small cycles with the same maximum strain as the overload cycle.  An effective strain-life curve is determined for the small cycles, and then compared to results obtained under fully reversed constant-amplitude conditions.  Fatigue testing was also performed under constant amplitude conditions for comparative purposes.

No. 28 Fig. 1 - New.jpgFigure 1

Figure 2 shows a composite plot of both the constant amplitude and overload results obtained for all three sulfur levels.  The data for Iteration No. 116 (0.004%S) is designated as “U Lo S”, the data for Iteration No. 117 (0.012%S) is designated as “Lo S”, and the data for Iteration No. 118 (0.077%S) is designated as “Hi S”.

The strain-life curves obtained at constant amplitude show results similar to what have been reported in previous posts.  The results at 0.004% sulfur are close to those at 0.012% sulfur, with the 0.012% sulfur results being slightly below those at 0.004% sulfur.  The fatigue results at 0.077% sulfur are significantly below those obtained at the other sulfur levels.

The overload fatigue results mirror the constant-amplitude data at somewhat higher-strain amplitudes.  The data are very close together at 0.004% sulfur and 0.012% sulfur, and the results at 0.077% sulfur are lower.   As strain amplitude is reduced in the long-life regime, the effect of sulfur level on overload results is minimized.

no-28-fig-2-newFigure 2

These results agree with data reported in earlier posts.  Fatigue performance is reduced if loading occurs transverse to elongated manganese sulfides formed during hot working.

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Effect of Overloads on the Fatigue Performance of a Forged Microalloyed Steel

In an earlier post, the fatigue properties of a microalloyed steel, in the form of hot-rolled bar stock, were compared with the properties of the same microalloyed steel in the form of forged crankshafts. In the most recent post, the effects of overloads on the fatigue properties of the microalloyed steel hot-rolled bars were examined. In this post, the results of further studies, which were conducted to evaluate the effect of overloads on the fatigue properties of the microalloyed steel forged crankshafts, are presented. These results are also compared with those obtained from the hot-rolled bars.

The chemical compositions and mechanical properties of both the hot-rolled bars and the forged crankshafts were reported in the earlier posts. However for convenience, they are shown again here in Tables 1 and 2 below.

blog-post-27-table-1-and-2

The same steel grade was used for both product forms, and the mechanical properties are similar.

Similar to what was noted in the most recent post, to simulate the effects of overloads on the fatigue performance of the crankshafts, a fatigue testing protocol was implemented in which high-amplitude cycles are inserted between groups of low-amplitude cycles.  The test protocol is shown schematically in Figure 1.  As can be seen, the load history consists of repeated blocks, each consisting of one fully reversed overload cycle and a series of small cycles with the same maximum strain as the overload cycle.  An effective strain-life curve is determined for the small cycles, and then compared to results obtained under fully reversed constant-amplitude conditions.

no-27-fig-1-newFigure 1

The comparative strain life fatigue properties for the forged crankshafts for both constant amplitude and overload testing are shown in Figure 2.

no-27-fig-2-newFigure 2

As can be seen, the effects of inserting overloads into the loading history has the same effect of reducing fatigue life of the forged crankshafts as was observed earlier for the hot-rolled bars.  The data from the hot-rolled bars was combined with the data from the forged crankshafts shown in Figure 2 to produce the comparative graph shown below in Figure 3.

no-27-fig-3-newFigure 3

This fatigue life as a result of the overloads is quite similar for both the hot-rolled bars and the forged crankshafts, with a possible slight advantage to the hot-rolled bars.  As was noted earlier for the constant-amplitude testing, the hot-rolled bars showed a slight improvement over the forged crankshafts in the long-life region.  This was attributed to the slightly higher hardness and tensile strength for the hot-rolled bar stock.  In future posts, the effects of overloads will be examined for other combinations of steel grade and processing.

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Effect of Overloads on Fatigue Performance

Strain-life fatigue data are most often developed under conditions of steady state constant amplitude. This provides a means of comparing the fatigue performance of various combinations of steel grade and processing. Many components, however, often experience random “spike” loads, or “overloads”, which are over and above those encountered during otherwise constant amplitude conditions. Automotive suspension and transmission components would be typical examples.

To simulate the effects of such overloads on fatigue performance, a fatigue testing protocol was implemented in which high amplitude cycles are inserted between groups of low amplitude cycles. The test protocol is shown schematically in Figure 1. As can be seen, the load history consists of repeated blocks, each with one fully reversed overload cycle and a series of small cycles with the same maximum strain as the overload cycle. An effective strain-life curve is determined for the small cycles, and then compared to results obtained under fully reversed constant amplitude conditions.

No. 26 Figure 1.jpgFigure 1

In the previous post, strain-life data were presented for a microalloyed steel, 1538MV. Data from both hot rolled bar stock and forgings were discussed. The hot rolled bar stock product form, chemical composition is shown in Table 1, and mechanical properties are shown in Table 2.

blog-post-26-table-1

blog-post-26-table-2

Comparative strain-life fatigue properties for both constant amplitude and overload testing are given in Figure 2.  The below graph presents the strain-life curve for the constant amplitude data and the effective strain-life curve developed from the load history (Figure 1).

No. 26 Figure 2.jpgFigure 2

It’s evident the insertion of overload cycles into the load history results in a significant reduction in fatigue life. Thus the performance of components subject to random overloads during fatigue loading is likely to reduce. There will be more discussion of this effect in future posts.

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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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