Bar Steel Fatigue Blog

Comparison of the Case and Core Properties of Induction Hardened Steels

Tue, 20 Jul 2010 09:34:00 -0400

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 Tensile Reduction BHN

Strength Strength in Area, %

MPa MPA

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.

Looking at a Comparison of As-Rolled and Normalized Properties

Mon, 21 Jun 2010 10:36:00 -0400

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 Tensile Reduction BHN

Strength Strength in Area, %

MPa MPA

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.

Figure 1 (Click here for larger figure)

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.

Does Grain Refining Practice Affect Fatigue Properties?

Fri, 04 Apr 2008 09:37:00 -0400

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.

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.

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.