Microalloyed steels are often used in the as-formed or as-forged condition in applications where the use of medium carbon steels would require that the component be quenched and tempered. Microalloyed steels, which contain small additions of niobium and/or vanadium, therefore offer the potential for saving the cost associated with heat treatment.
The AISI Bar Steel Fatigue Database provides information on both heat treated medium carbon steels and as-hot formed microalloyed steels, which permits a comparison of fatigue properties. Shown in the table below are the mechanical properties and hardness values for three steel grades taken from the database.
The SAE 1038 steel contained 0.40% carbon and 0.80% manganese. The SAE 1045 steel contained 0.44% carbon and 0.73% manganese. The mechanical properties and fatigue data were obtained from quenched and tempered steel bars. The 15V24 steel contained 0.28% carbon, 1.36% manganese and 0.13% vanadium. Mechanical properties and fatigue data were obtained from automotive control arms, which had been hot formed and control cooled. The microstructures of the SAE 1038 and SAE 1045 grades were mixtures of ferrite, bainite and martensite. The microstructure of the 15V24 steel was ferrite and pearlite.
Figure 1 shows the strain-life curves for all three steel grades. The strain-life curves for Iteration Nos. 20 and 26 give the fatigue properties for quenched and tempered SAE 1038 and SAE 45 respectively. The strain-life curve for Iteration No. 28 shows the fatigue properties for 15V24.
It can be seen that roughly comparable fatigue performance is achieved by all three steels. The strain-life curves for the quenched and tempered SAE 1038 and SAE 1045 grades show somewhat higher performance in the short life regime compared to the 15V24. However the 15V24 exhibits somewhat higher performance in the long life regime.
In general, the data shows that the microalloyed steel in the as-hot formed and controlled cooled condition could be substituted for the quenched and tempered medium carbon steels without degradation of fatigue performance. The decision to substitute can be made based upon comparison of other properties as well as cost.