Hardness versus Fatigue Strength

In the previous article we examined the relationship between hardness and tensile strength for the AISI fatigue database iterations through 141. As the hardness increased the tensile strength also increased for a majority of the steels and the relationship was linear. However, with the high hardness, high carbon, carburized case steels there was a significant variation in strength in the 600-750 Brinell hardness range. The variation in strength was 881-2227 MPa, which is a difference of 2.5:1.

In this article we examine the relationship between hardness and fatigue strength for the same samples. The fatigue strength is defined as the stress level where one million cycles is achieved. The data is shown in Figure 1. The curve looks much like the previous hardness versus strength curve. There is a linear correlation between hardness and fatigue strength to 600 Brinell. However, there seems to be more scatter or variation in the hardness versus fatigue strength curve. At a hardness of 600-750 Brinell there is a large variation in fatigue strength primarily due to the carburized case samples. The fatigue strength range is 235-819 MPa, which is a difference of 3.5:1 between the high and low iterations.

SMDI Blog 36 Figure 1Figure 1

The fatigue ratio is the fatigue strength divided by the ultimate strength. The fatigue ratio versus hardness data is shown in Figure 2. The fatigue strength does not appear to be a fixed percentage of the ultimate tensile strength throughout the entire hardness range. At 200 Brinell the fatigue ratio is approximately 0.45, while at 700 Brinell it is 0.35. As the hardness and strength increases the fatigue strength/ultimate strength becomes a smaller ratio.

figure 2Figure 2

 

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Hardness versus Strength

There is a very good relationship between hardness and strength for steel. If the hardness is known the tensile strength can be estimated, and the reverse is also true. This relationship is valid for tensile strength but not for yield strength. This relationship is true for most any kind of steel, whether it is as rolled, forged, heat treated or cold worked. A graph of this relationship is provided in SAE J413.

The relationship of hardness versus strength for most of the iterations through 141 is shown in Figure 1. Three different classes of steels are shown, through-hardened, induction-hardened case and carburized case. The through-hardened steels are primarily made up of as rolled, forged and cold worked low to medium carbon steels which may or may not have been quenched and tempered. With these steels there is a linear relationship between hardness (from 130 to 650 Brinell) and tensile strength, which is in good agreement with SAE J413. With the induction-hardened case steels there are only three data points. These three steels were SAE 1050M, SAE 1552 and SAE 1070. Two of the data points seem to follow the same curve as the through-hardened steels while one, the SAE 1552, is significantly lower in strength. In a previous blog article, “Strength and Fatigue Life versus Carbon Content at High Hardness” it was determined the reduced properties were likely caused by the large grain size present in these samples. With the carburized-case steels there is a tremendous variation in strength at the same hardness. At 600-750 Brinell there is a difference of 2.5:1 between the maximum and minimum strength of the test iterations. There are about 4 or 5 data points that fall within the through-hardened curve while the majority is much lower. It is typically assumed the carburized case has high strength because of the high hardness and the core is lower in strength because it is softer. However, the data indicates in many cases the carburized case is equal in strength to the softer core. At the very low end it may only be equal to a core with a hardness of about 250 Brinell.

What is the reason for this behavior of the carburized case? Again, from the previous article referenced above we know that both a large grain size and a high carbon content can decrease strength at a hardness of 60 HRC. Certainly, many of the carburized case samples have both of these conditions as some were through-carburized with very long cycle times. However, others were case hardened to produce a case/core composite at more typical cycle times. In future articles we will take a closer look at the carburized case samples to try to determine what other factors may be affecting the strength level, as well as what the relationship between hardness and fatigue looks like.

SMDI Blog 35 Figure 1Figure 1

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Effect of Intergranular Oxidation on Axial Fatigue Test Bar Performance

Two 100 pound heats of vacuum melted 8695 steel were made for iterations 40 and 41. This chemistry represents the case of carburized 8620 steel. The two heats were forged into bar stock from which axial test bars were machined. In order to produce intergranular oxidation (IGO), both sets of test bars were carburized. Iteration 40 was copper plated prior to carburizing to prevent carbon diffusion and any associated IGO. Both sets of bars were polished prior to testing. Figure 1 shows the IGO present at the surface in iteration 41.

SMDI Blog 34 Figure 1Figure 1

Table 1 shows the mechanical properties and hardness for both iterations, and Figures 2 and 3 show the stress-strain curves. The static strength of iteration 40 without IGO is about 50% greater than iteration 41 with IGO. The elongation or ductility is also greater. It would seem possible that this difference could be related to the presence of IGO, however there is also a difference in how the samples were processed.

Table 1

SMDI Blog 34 Table 1

SMDI Blog 34 Figure 2
Figure 2

SMDI Blog 34 Figure 3
Figure 3

Figure 4 shows a comparison of the fatigue curves for iterations 40 and 41. In the low cycle portion of the curve, iteration 40 has better fatigue life. This is typical for a sample with higher strength. However, in the high cycle portion of the curve, iteration 41 has the better fatigue life. If IGO had a negative effect on performance this is where it would occur. It can therefore be concluded IGO is not detrimental to the fatigue life of these samples.

SMDI Blog 34 Figure 4.jpgFigure 4

 

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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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