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The austenite grain coarsening behavior of low carbon (0.13% C) structural steels containing chromium and nickel singly or in combination were studied by heating the steels at successive high temperature in the austenite zone in the temperature range of 900-1100°C with an interval of 50°C. The carburizing technique has been adopted to reveal the prior austenite grain boundaries and mean linear intercept method was used to measure the austenite grain size.It was found that on heating the undissolved particles of chromium carbide, Cr2C refine the austenite grain size at temperature below 1000°C, but the effect decreases with increasing temperature. Nickel does not produce any austenite grain refinement. In the presence of nickel particles of chromium carbide are less effective than chromium carbide particles in the absence of nickel in the refinement of austenite grain size.

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Three 0.15% carbon steel samples containing small additions of vanadium and nitrogen singly or in combination have been carburized in a natural Titas gas atmosphere at a temperature of 9500C and a pressure of about 15 psia for time periods ranging from 1 to 5 hours and quenched in 10% brine from the carburizing temperature of 9500C after pre-cooling to 8600C in the furnace followed by tempering at a low temperature of 1600C. The structure and properties of the carburized and heat treated specimens were studied systematically by optical microscopy, surface hardness and microhardness measurements, X-ray diffractometry and impact tests. It was found that vanadium without nitrogen does not have any effect in the formation of retained austenite while vanadium with nitrogen is effective in promoting the formation of retained austenite in the case of carburized and hardened steels. It was also found that vanadium alone and vanadium with nitrogen refine the martensite platelets (needles) in the case of carburized and hardened steels, vanadium with nitrogen being more effective. Microhardness measurements have shown that vanadium improves the case hardness and the core hardness values; vanadium with nitrogen is more effective than vanadium alone in increasing the case hardness and the core hardness. The hardenability is found to increase with the increase of austenite grain size and with the extent of carbon penetration of the case of carburized steels. Vanadium as vanadium carbide, VC are detrimental to toughness and vanadium as vanadium carbonitride, V(C, N) are beneficial to toughness of the core of low carbon steels in carburized and hardened condition.

The effect of small addition of chromium and nickel alone or in combination on the transformation characteristic and ferrite grain size of low carbon (0.13%C) structural steels have been studied by cooling suitable steels at four different cooling rates ranging from 120°C/min to 3.6° C/min from temperatures giving a constant austenite grain size of 37 μm. Radio Frequency generator with control system was used for the heat treatment of the steel samples. Optical microscopy of the heat treated samples was carried out. Ferrite grain size was determined from the fictitious ferrite grain size measured by mean linear intercept method and the volume fraction of pearlite obtained by optical microscopy and point counting. It was found that although the heat treatment of the steels was started from a common austenite grain size, their subsequent ferrite grain size after cooling at the same cooling rate were not the same. Both chromium and nickel enhance the formation of Widmanstatten structure. But chromium is more effective than nickel in the formation of Widmanstatten structure. It was also found that the undissolved particles of chromium carbide (Cr2C) present during austenitizing have no role in determining the ferrite grain size. The precipitating particles of chromium carbide (Cr2C) are excellent ferrite grain size refiners. Nickel refines the ferrite grain size. In presence of nickel, Cr2C precipitates are less effective than Cr2C precipitates in absence of nickel in the refinement of ferrite grain size.