Fatigue Properties of Modified Steels Carburized at High Temperatures

The objective of phase XI is to compare the performance of carburized steel in bending and axial fatigue loading conditions. Carburization is a surface heat treatment process which adds carbon to the surface of a material to increase its hardness and other mechanical properties. Such heat treatments are typically performed on materials used in shafts and gear applications. SAE 8615 was chosen for this study as it is a common carburizing grade with good toughness.

A total of ten iterations of constant amplitude and overload axial and 4-point bending fatigue were performed for three steel conditions as shown in Table 1.

Table 1: Phase XI – Testing Methods

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* Simulated carburized core is defined as through carburized.

** Shallow case depth is defined as 10% of the gage radius is carburized.

*** Deep case depth is defined as 20% of gage radius is carburized.

The constant-amplitude axial fatigue tests provide information about the cyclic and fatigue behavior of a material.

Background on SAE 8615 Steel:

The SAE 8615 is principally a carburizing grade of steel. It finds medium-strength applications where high surface hardness and high core toughness are required. This alloy is used as forged shafts and bearings in automotive and aerospace industries.

  • Machinability: SAE 8615 is readily machinable following suitable cooling from forging or subsequent heat treatment.
  • Weldability: This alloy may be welded by normal fusion methods prior to carburizing, hardening or tempering.

 

Monotonic Tension Tests:

Two monotonic tension tests are conducted to ensure accuracy of the results. The engineering stress-strain and true stress-strain curves are obtained from tension tests (Fig. 1 and Fig. 2) from which various properties, as given in Table 2, are calculated.

Fig1

Figure 1: Engineering Stress – Strain Curves

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Figure 2: True Stress – Strain Curves

 

Table 2: Monotonic Tensile Properties

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Cyclic Tests:

When a material is loaded in a cyclic mannerit first shows a transient response for a number of loading cycles until stabilizing for a time-constant response. This phenomenon is called cyclic stabilization.

In this first of four blogs, we will evaluate the response of SAE 8615 steel in first of three conditions – the simulated carburized core condition. Fourteen constant-amplitude strain-controlled axial fatigue tests were conducted on the heat treated coupons, which were loaded at 7 different strain amplitudes ranging from 0.2% to 2.0%. The displacements were measured using an extensometer and then converted to strain values. The plot of stress amplitude vs reversals to failure obtained from these tests tells us if a material cyclically hardens or cyclically softens. Figure 3 shows a comparison between the monotonic tensile curve and the cyclic stabilized fatigue curve, and indicates SAE 8615 cyclically softens.

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Figure 3: Composite Plot of Monotonic and Cyclic Stress-Strain Curves

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Figure 4: True Stress Amplitude vs Number of Cycles

For any given strain amplitude the true stress amplitude drops with number of cycles until the material finally fails (denoted by straight vertical lines), as shown in Figure 4. Two exceptions are for strain amplitudes of 0.25% and 0.3%, where the stress amplitude is 500 MPa and 560 MPa respectively. At these strain amplitudes, constant stress amplitude is developed and the samples do not fail when cycled up to 1×106 cycles. This is the fatigue strength or endurance limit of the material.

Once the cyclic stabilization occurs, the cyclic properties of a material are determined. The cyclical properties for SAE 8615 steel in simulated carburized core condition are shown in Table 3. These properties will be useful to engineers when they are designing a component for fatigue.

Table 3: Cyclical Properties of SAE 8615 in Simulated Carburized Core Condition

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It is also interesting to analyze is the strain-life curve of the material in constant-amplitude axial fatigue tests. As shown in Figure 5, the green dots and the corresponding curve show the true strain amplitude vs reversals to failure of SAE 8615 steel in simulated carburized core condition. ‘Reversals to failure’ is a term used in fatigue instead of cycles; 1 cycle = 2 reversals. From Figure 5, we can determine the fatigue life of a material.

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Figure 5: True Strain amplitude vs Reversals to Failure

Bending Fatigue Tests:

4-point bending fatigue tests were conducted after axial fatigue testing. The true strain amplitude vs reversals to failure results are shown in Figure 6.

fig6.png

Figure 6: True Strain Amplitude vs Reversals to Failure in a 4-point Bending Test

Though a curve trend is not present in the above graph, a select point comparison between the curves from axial and 4-point bending tests can be extrapolated, as shown in Table 4.

Table 4: Fatigue Life Comparison of Two Cases

table4.png

As seen from the results above, the SAE 8615 shows similar behavior in both constant-amplitude axial and 4-point bending fatigue testing. The axial fatigue curve is more linear than the four-point bending fatigue curve where the strain amplitude at both 1×104 and 1×105 cycles is lower in four-point bending fatigue than in axial fatigue.

This blog reviewed the axial and four-point bending fatigue of SAE 8615 steel in simulated carburized core condition. The next blogs in this series will review the SAE 8615 steel performance in shallow- and deep-case depth conditions in axial, overload axial, 4-point bending and overload 4-point bending conditions.

Stay tuned for some really interesting fatigue life analyses!

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