Spheroidization of a medium carbon HSLA fastener grade steel
Medium carbon, low alloy, high-strength steels containing Ni, Cr, and Mo are used for the fabrication of various types of high-strength fasteners required for launch vehicle structures. Cold heading properties and response to hardening and tempering are essential requirements of the material for fabrication and application, respectively. In the absence of a suitable spheroidizing cycle for cold heading of medium carbon 3.8 Ni, 1.8 Cr, 0.3 Mo steel, experiments were carried out with theoretical understanding of the alloy system. This paper presents the experiments and results obtained after spheroidizing treatments carried out to optimize the process. Hardness measurements were carried out and microstructural studies were made to assess the spheroidization process and to determine the optimal spheroidization parameters, to achieve hardness in the range of 200-220 BHN.
Spheroidized microstructures are classically obtained either by prolonged isothermal treatment of austenite structure below the lower critical transformation temperature or by overtempering a martensite structure. In both cases, carbides would coalesce to globular particles in a ferritic matrix.
High-strength, low alloy (HSLA) steels containing Ni-Cr-Mo reportedly respond well to spheroidizing heat treatment. However, the time increases due to various metallurgical reasons such as alloying elements, which reduce the lower critical temperature resulting in longer spheroidizing time due to low diffusion rates at lower temperature. The spheroidization process can be completed in less time by increasing the temperature. However, it may result in the reappearance of dissolved carbides as lamellae during cooling.
Several experimental trials were carried out to get the maximum degree of spheroidization by both the routes, i.e. through prolonged soaking below the lower critical temperature and through over-tempering of supersaturated martensite (see table). Hardness measurements and microstructural characterization were performed after each treatment, and subsequent experiments were decided accordingly. In this way, a total of eight cycles have been tried during the course of the experiment.
Results and Discussions
The first six cycles (C1 to C6) were tried through promoting the lamellar reaction to form pearlite and coalescence of carbides into spheroids by prolonged isothermal soaking. Desirable results were obtained with respect to hardness in the range of 220 to 240 BHN. However, C1 to C6 cycles led to a relatively longer transformation time, which may be due to the slower rate of diffusion of solute atoms present in the carbides as compared to carbon atoms, and this becomes the rate-controlling factor for spheroidization.
An alternate approach was adopted (Cycle C7-C8) to obtain a martensitic structure prior to spheroidization, as it would help in the faster and uniform nucleation of spheroids in an over-tempered microstructure. It also resulted in the 220 to 240 BHN hardness of steel. Microstructural observation of the sample was very much similar to the first approach. This substantiated the acceleration of spheroidization in an acicular martensite structure during over-tempering.
Prolonged annealing below the lower critical temperature was found to be better than intercritical annealing for this alloy. However, the long soaking time is not the best production option.
By over-tempering, the desired level of spheroidization could be achieved in full martensite structure, which could be obtained even without sub-zero treatment.
The alloy was found to be of air hardening type, and the desired results could be obtained by air-cooling from austenitizing the temperature prior to over tempering. Cycle C8 is the best commercial option.