The Effect of Nitrogen on the Mechanical Behaviour of Cold-Worked Austenitic Stainless Steel Rod

Abstract

The effect of nitrogen in solid solution on the work hardening rate of austenitic stainless steel is well known and forms the basis for production of many engineering components. An obvious application is in rod and wire production while in bioengineering, for example, stainless steel prostheses derive strength from the combined effects of nitrogen in solid solution and cold forging, while improved corrosion resistance is also gained from nitrogen in solid solution. In the present work a requirement was identified for stainless steel rod with improved surface hardness supported by a strengthened rod core. This geometry is easily utilised in both theoretical modelling and experimental investigation of a solid-state processing route. A method of fabrication was proposed comprising of nitriding of the surface of austenitic steel rod to a controlled depth and subsequent swaging or wire drawing to provide cold deformation. These two steps can be assimilated into a continuous industrial process to produce a composite rod structure with high surface hardness and a strong core. The evolution of microstructure and properties in a pilot-scale process are described together with the variation of properties of the nitrided rod with different degrees of cold work. The composite rods are tested for mechanical behaviour and a model is developed to describe the response to loading. The objective is to devise a predictive model which will allow process development without the need for detailed experimental work at every stage. The model combines a diffusion calculation to predict the penetration of nitrogen during nitriding with a mechanical model to give the response to loading.