Influence of Alloying Elements on the Annealability of Carburizing Steels J. M. Tartaglia, T. Wada, D. E. Diesburg, G. T. Eldis SUMMARY The effects of manganese (0.6-1.0 wt.%), nickel (0-1.8%), chromium (0.5-1.0%), and molybdenum (0.2-0.7%) on annealability of 0.2% C carburizing steels were determined and the results have been expressed in a regression equa- tion. Annealability was defined as the time required to reach a hardness of 200 HVlO during isothermal transfor- mation studies. The regression equation relating annealing time to hardenability and composition showed that substitution of the ferrite-stabilizing elements Cr and Mo for the austenite-stabilizing elements Mn and Ni is gener- ally an effective means of reducing the annealing time while maintaining constant hardenability. These results can best be utilized when designing high-hardenability steels. Exam- ples are given of steels having hardenabilities with DJ values in the range of 130-155 mm (5-6 in.) having annealing times ranging from 1.5 to 12 h. INTRODUCTION Carburizing steels are typically softened by annealing to enhance their formability and machinability prior to carburizing. Often the steel$ will be annealed more than once between forming operations. Shortening the annealing time required to soften the steel can lead to significant cost savings. The ease of achieving the required softness, i.e., the time required, is generally referred to as annealability. Of course, the benefits gained by achieving shorter annealing times can best be realized when the annealing times required far exceed that minimum time required to heat a furnace and load of parts. For example, decreasing annealing time by 30 min compared to steels that already soften in 1 h is of little practical importance if the annealing operation itself already requires 2 h. On the other hand, decreasing the annealing time from 6 to 2 h would be considered very sig- nificant. An inverse relationship between hardenability and annealability is usually anticipated, i.e., a steel with high hardenability normally has poor annealability. However, steels with different alloy compositions but equal harden- abilities may have different annealabilities. The present study was conducted to evaluate the relative effects of manganese, nickel, chromium, and molybdenum on anneal ability and develop a regression equation relating annealing time to hardenability and composition. Each of the four elements was first investigated at each of two levels as follows: 0.6 and 1% Mn; 0.95 and 1.8% Ni; 0.55 and 1% Cr; and 0.2 and 0.4% Mo. In addition to the 16 heats that covered all combinations of the above dual levels, eight steels with no nickel and/or molybdenum and seven steels with either higher molybdenum or other differences in alloy content were included. Since 200 HVI0 (approxi- mately 91.5 HRB)l is a suitable hardness for machining 30 and the minimum isothermal transformation time required to reach 200 HVI0 (t200) corresponded well to the mini- mu.m time for the completion of transformation to pearlite, t200 was chosen as the index of annealability. EXPERIMENTAL PROCEDURES Materials The chemical compositions of the test steels are shown in Table I. 'The first 24 steels were prepared by induction melting of 10 heats in a vacuum furnace backfilled with argon. Each of these 23-27 kg (50-60 lb) heats was cast into 90-mm (3.5-in.) diameter ingot molds with ladle addi- tions of alloy between ingot casts to obtain the 24 chemistries desired. Ingots of Steels 1 through 24 were forged into 30-mm (1.25-in,) diameter and 20-mm W.8-inJ diameter bars. Steels 25 through 31 were induction melted in air and aluminum deoxidized. Each of these 23 kg (50 lb) heats was forged into 30-mm (1.25-in.) square bar stock. All the bars were normalized at 925°C (1700°F). End·Quench Hardenability Jominy hardenability test specimens were machined from normalized 30-mm (1.25-in.) diameter bars. The Jominy specimens were austenitized for 30 min before end-quench- ing in accordance with ASTM Standard A255. The austen· itizing temperature was 900°C (1650°F) for the vacuum! argon·melted steels 1·24, and 925°C (1700°F) for the air· melted steels 26·31. Two parallel flats were ground on each bar and Rockwell C hardness determined as a function of the distance from the quenched end. The ideal critical diameter (DI) was determined from the Jominy curve by relating the quenched-end hardness (99% martensite) to the expected hardness at 50% martensite,2 locating the position on the Jominy curve (distance from the quenched end) corresponding to this expected hard- ness, and correlating this distance to the diameter (DI) of a cylinder whose center would experience the same cooling rate when subjected to an ideal quench. 3 The ideal critical diameter was also calculated using known multiplying fac- tors for the various alloying elements. 4 Isothermal Transformation Wafers were cut from the normalized 20-mm W.8-in.) diameter and 30-mm (1.25-in.) square bar stock and ground to a thickness of 3-mm (0.125-inJ, austenitized in a lead bath for 30 min at 900°C (1650°F), and transferred to another lead bath stabilized at selected temperatures between 600 and 725°C (1110 and 1340°F). After holding at the isothermal transformation temperature for a selected period of time between 0.25 and 32 h, the wafers were water quenched. . JOURNAL OF METALS· May 1982