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Bar Steel Fatigue Blog
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In recent postings, the strain-controlled fatigue properties of the high-hardness case and the lower-hardness core of carburized low-alloy steels were compared. 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. It was shown that at short lives, the softer core out-performed the high-hardness case. At long life, the fatigue properties of the high-hardness case were found to be superior.
This work was expanded to examine the fatigue properties of specimens carburized to a specific case depth, and to compare them with the properties developed through separate heat treatments simulating case and core.
SAE 8620 was selected and two sets of specimens were heat treated to simulate case and core respectively, as described above. In addition, a third set of specimens was carburized to develop a case depth of 0.03 inches. The simulation heat treatments of the first two sets of specimens resulted in one set with a high-hardness martensite case, and a second set with a lower hardness, mixed-bainite/martensite core. A composite microstructure was developed in the third set of specimens, which consisted of a high-hardness martensite case near the surface, and a lower hardness, mixed microstructure at the center.
The mechanical properties and hardness obtained for the three conditions are as follows:
Location Yield Str. Tensile Str. Red. in Area HRC
MPa MPa %_______
Core 1420.0 1683.0 36.2 45
Case 1125.3 1868.7 0.7 58
Case/Core 1356.6 1764.1 13.9 (See Fig. 1)
Composite
Figure 1 shows the hardness profile developed for the case/core composite specimens. The hardness at the surface is about 58 HRC and the hardness of the core is about 45 HRC.
Figure 1 (Click here for larger image)
Figure 2 shows the strain-life curves for all three conditions. The strain-life curve for Iteration No. 39 shows the fatigue properties of the simulated core, and the strain-life curve for Iteration No. 71 shows the properties for the high-hardness simulated case. The fatigue properties for the case/core composite specimens are given by the strain-life curve for Iteration No. 62.
Figure 2 (Click here for larger image)
The data show that, as has been demonstrated in earlier postings, the high-hardness simulated case exhibits better long-life fatigue properties. At short life however, the data shows that the simulated core exhibits superior properties to those of the case, which suggests that the case may be vulnerable to cracking due to overloads. A comparison of this data with that of the case/core composite shows that at long life, the case/core composite exhibits fatigue properties very close to those shown by the high-hardness simulated case. This suggests that the long-life properties of a carburized part are controlled largely by those of the case.
At short life however, the fatigue properties of the case/core composite are between those of the simulated case and core. This indicates that at short life, the properties of a carburized part will be a combined function of case and core. The case core/composite data are superior to those of the high-hardness case, but do not approach those of the lower hardness core.
In the previous posting, the strain-controlled fatigue properties of the high-hardness case and low-hardness core of carburized SAE 4620 steel were compared. It was shown that at short lives, the softer core exhibited better properties than the high-hardness case. At long life, the fatigue properties of the high-hardness case were found to be superior.
Again using SAE 4620 steel, AISI undertook a study to determine what if any changes in fatigue properties could be expected by changing continuous casting practice during steel manufacturing. The steel described in the previous posting was continuously cast into 175mm by 175mm billets, and hot rolled to 50.8mm diameter bars. For comparison, a second heat of SAE 4620 was evaluated that had been continuously cast to 279mm by 375mm blooms, and hot rolled to 47.6mm diameter bars. For the billet cast steel the reduction in area during hot rolling was 93% and for the bloom cast steel it was 98%. In both instances, 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 mechanical properties and hardness values obtained for the case and core, both for the billet cast and bloom cast steel, are shown below:
Casting Location Yield Str. Tensile Str. Red. in Area HRC
Practice MPa MPa %__________
Billet Core 891.5 963.7 61.9 29.6
Billet Case 1169.1 1775.6 1.4 54
Bloom Core 687.5 997.8 58.2 29
Bloom Case 1316.6 2226.9 4.0 59
In both cases, the case microstructure was 100% martensite, and a mixed bainite-martensite microstructure was developed in the core.
Figure 1 (taken from the previous posting) shows the strain controlled fatigue properties for both the case and the core of the billet cast SAE 4620. The strain-life curve for Iteration No. 53 shows the fatigue behavior of the core, and the strain-life curve for Iteration No. 54 shows the behavior for the high-hardness case.
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Figure 2 shows the fatigue properties for both the case and core of the bloom cast SAE 4620. The strain-life curve for Iteration No. 47 shows the fatigue properties for the core, and the strain-life curve for Iteration No. 48 shows the properties for the high-hardness case.
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The two graphs show that both billet cast and bloom cast carburized SAE 4620 behave in a similar fashion. At short life, the core exhibits superior fatigue properties, whereas at long life the properties of the high-hardness case exceed those of the core. For each set of data a cross over occurs near 104 reversals. The data also suggest that irrespective of the casting practice, the high-hardness case may be vulnerable to cracking as a result of periodic overloads.
Figure 3 compares the fatigue properties of the core for both bloom- and billet-cast SAE 4620, and Figure 4 gives the same comparison for the high-hardness case. In Figure 3 the strain-life curve for Iteration No. 47 shows the fatigue properties for bloom cast SAE 4620, and the strain-life curve for Iteration No. 53 shows the properties for billet-cast SAE 4620. In Figure 4, the bloom cast results are given by Iteration No. 48, and the billet cast results by Iteration No. 54.
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Figure 4 (click here for larger image).
It can be seen that the core properties for the two casting practices are nearly identical. The case properties of bloom cast steel, however, do show a slight advantage over the billet cast steel in the very short life, high strain amplitude region. Thus it appears that at all but the highest strain amplitudes, billet and bloom cast carburized SAE 4620 will perform equally. At very high strain amplitudes, the carburized case of the bloom cast steel appears to be somewhat less vulnerable to cracking that may result from "spike" loading.
In recent postings, the strain-controlled fatigue properties of the high-hardness case and low-hardness core of induction hardened steels were compared. It was shown that at short lives, the softer core outperformed the high-hardness case. At long life, the fatigue properties of the high-hardness case were found to be superior.
Case hardening by carburizing presents an opportunity for a similar comparison. This process results in a component with superior wear resistance along with excellent mechanical properties and toughness. During carburizing, a component is heat treated in a carbon-bearing atmosphere to permit diffusion of carbon into the surface of the part. A high-carbon case is developed, the depth of which is a function of time and temperature during heat treatment. At the end of the process, the component is quenched and tempered resulting in a very high hardness case and a lower hardness core.
AISI has examined the fatigue properties of both the case and core for a number of steel grades used in carburizing applications. In this posting, the properties of SAE 4620 are presented. 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 following chart shows the mechanical properties and hardness obtained for the case and core:
Location Yield Str. Tensile Str. Red. in Area HRC
MPa MPa %_________
Core 891.5 963.7 61.9 29.6
Case 1169.1 1775.6 1.4 54
The case microstructure was 100% martensite, and a mixed bainite-martensite microstructure was developed in the core. The differences in the mechanical properties and hardness between case and core are quite evident.
Figure 1 (below) shows the strain controlled fatigue properties for both the case and the core.
The strain-life curve for Iteration No. 53 shows the fatigue behavior of the core, and the strain-life curve for Iteration No. 54 shows the behavior for the high-hardness case. As was shown for induction hardened steels, in the short life regime, at high strain amplitudes the softer core exhibits better fatigue properties than the case. In the long life regime, where strain amplitudes are lower, the case shows superior fatigue properties to the core. A cross-over point can be seen between 103 and 104 reversals.
The data show that as expected, high hardness such as that shown in the case, results in better long life fatigue properties. At short life however, the data suggests that the high hardness case may be more prone to crack initiation where occasional overloads are encountered.
In the last posting, the fatigue properties of surface induction hardened 1050 axle shafts were presented. Strain life data showed that in the short life regime, the fatigue properties of the low hardness core were better than those of the case. In the long life regime the fatigue properties of the high hardness case were superior to those of the core.
Here we would like to look at additional data that has been developed on induction hardened SAE 1070 steel. In this study, the case and core properties were simulated using 50mm diameter steel bars. As-hot rolled bars were used to simulate an induction hardened core, and additional bars were through-induction hardened to simulate the high hardness case.
The mechanical properties and hardness obtained for the simulations of core and case were as follows:
Location Yield Str. Tensile Str. Red. in Area BHN
(MPa) (MPa) %
Core 520.0 659.0 36.2 280
Case 1950.0 2069.0 2.3 613
The core exhibited a ferrite-pearlite microstructure and the high hardness case contained a mixture of martensite, bainite and a small amount of pearlite.
The strain-controlled fatigue properties determined for both the case and the core are shown in Figure 1.
The strain-life curve for Iteration No. 36 shows the fatigue behavior of the core, and the strain-life curve for Iteration No. 37 shows the behavior for the high hardness case. As was observed for the SAE 1050 steel in the previous post, the softer core exhibits better fatigue properties than the case in the short life (high strain amplitude) regime. Conversely, in the long life regime (low strain amplitudes), the properties of the high hardness case are superior to those of the core. A cross-over point occurs between 104 and 105 reversals.
Additional comparisons of the case and core of surface hardened steels will be included in future postings covering the properties of carburized low alloy steels.
Medium and high carbon bar steels are often used in the induction hardened condition for applications such as axles and shafts. Induction hardening is a surface hardening heat treatment whereby a component is rapidly heated for a short period of time in an induction coil and then quenched. This results in a high-hardness, wear resistant case and a softer core. Of interest are the comparative fatigue properties of the hardened case versus the softer core. AISI obtained axle shafts that had been hot forged, cold extruded and induction hardened. The steel grade was SAE 1050.
The mechanical properties and hardness obtained for the case and core were as follows:
Location Yield Str., MPa Tensile Str., MPa Red. in Area, % BHN
Core 460 828.5 34.1 220
Case 2100 2360 14.7 536
The core exhibited a ferrite-pearlite microstructure, and the high hardness case was 100% martensite. The differences in the mechanical properties and hardness of the case and core are (as might be expected) quite significant.
The strain-controlled fatigue properties determined for both the case and the core are shown in Figure 1.
Figure 1 (click here for larger image).
The strain-life curve for Iteration No. 4 shows the fatigue behavior of the core; the strain-life curve for Iteration No. 5 shows the behavior for the high-hardness case. In the short life regime (at high strain amplitudes), the softer core exhibits better fatigue properties than the case. In the long life regime where strain amplitudes are lower, the case shows superior fatigue properties to the core. A cross-over point can be seen at approximately 104 reversals.
High hardness is generally considered to result in better fatigue properties at long life, and this is confirmed in Figure 1. This data also shows however that at high strain amplitudes a high hardness exhibits a greater tendency toward crack initiation. This suggests that the case of induction hardened shafts may be vulnerable to crack formation under conditions where "spike" loading occurs.
While medium carbon bar steels are often used in the as-hot rolled condition, some applications call for normalizing the hot rolled product. Normalizing consists of re-austenitizing the steel followed by ambient air cooling. This often results in improvements in ductility and notch toughness.
As part of the development of the AISI bar steel fatigue database, the properties of SAE 1541 steel were examined in the hot rolled and normalized conditions. The hot rolled bars were given a slight cold sizing treatment; the normalized bars were subjected to austenitizing at 900°C and air cooled.
The mechanical properties obtained for the two conditions were as follows:
Condition Yield Str., MPa Tensile Str., MPA Red. in Area, % BHN
As-Rolled 461.0 905.5 41.7 195
(Cold Sized)
Normalized 471.2 783.2 55.1 180
Normalizing resulted in a slight increase in yield strength, a reduction in tensile strength and hardness, and improved ductility.
Both the as-rolled and normalized conditions exhibited ferrite-pearlite microstructures.
The strain-controlled fatigue properties determined for both conditions are shown in Figure 1.
It can be seen that Iteration No. 1 gives the fatigue results after normalizing, and Iteration No. 2 shows the fatigue properties in the as-rolled condition. The curves drawn through the data points for each iteration were calculated from their respective strain-life equation. As can be seen, the fatigue properties for both conditions are very similar.
In the case of the long life regime, the curves show the as-rolled SAE 1541 as having somewhat better fatigue performance than the normalized SAE 1541. A calculation of the fatigue strengths at one million cycles from the strain life equations results in values of 312 MPa for the as-rolled condition, and 260 MPa for the normalized condition This might be expected, since the as-rolled condition exhibited slightly higher tensile strength and hardness.
However, as can be seen from the actual data points, the difference in the fatigue performance between the two conditions is quite modest. Thus application considerations should focus primarily on mechanical property requirements, with fatigue performance being a secondary consideration.
The steel-making practices followed for grain refinement can vary, particularly with respect to the grain refining element selected. Aluminum, niobium and vanadium are all used for grain refinement in medium carbon steels. AISI has examined this issue for SAE 1141 steel in both the normalized and quenched and tempered conditions.
Figure 1 shows strain-life fatigue data for three test iterations in the normalized condition. Iteration 11 was grain refined with aluminum, Iteration 13 with niobium, and Iteration 15 with vanadium. The hardness values fell in a fairly narrow range, 199-223 BHN, and the microstructures were ferrite-pearlite.
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Figure 1 (Click here for larger figure)
As can be seen, there is little variation in the strain-life curves for the three sets of data. This suggests that any of the three grain refining elements can be employed without worrying about differences in fatigue properties.
Figure 2 shows results obtained for SAE 1141 steel in the quenched and tempered condition. Iteration 12 was grain refined with aluminum, Iteration 14 with niobium, and Iteration 16 with vanadium.
In these cases there was more variation in hardness than in the normalized condition. Iteration 12 and 16 exhibited hardness values of 277 and 252 BHN and the microstructures were mostly martensite. Iteration 14 had a hardness value of 241 BHN, and the microstructure contained a significant amount of bainite and some ferrite.
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Figure 2 (Click here for larger figure)
There is somewhat more scatter in the strain-life fatigue data, however it can still be concluded that the various grain refining elements can be used interchangeably.
The data does indicate that even where fatigue performance is a consideration for a given application, grain refining elements can be selected based on other factors such as steel manufacturing preferences, economics, etc.
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