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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