Date of Award

December 2019

Degree Type

Thesis

Degree Name

Master of Science

Department

Engineering

First Advisor

Pradeep Rohatgi

Committee Members

Benjamin Church, Nathan Salowitz

Keywords

Alloying, Casting, Stainless Steel, Surface Alloying

Abstract

Casting is one of the oldest manufacturing processes in practice and the growth of casting industry through the ages has led to scientific leaps in the field of metallurgy. As we look into the history of metal castings, it is prominent that a steady growth in the knowledge leading to rise of newer casting techniques has caused an exponential increase in the way we can use metal castings to improve our lives. The surface alloying of mild steel to compositions targeted towards stainless steel compositions was performed in this study by sand casting process. Sand casting is one of the cheapest methods of production of components used in industries ranging for kitchen utensils, automobile to the water and piping industry. All of the industries require superior castings with a surface that would resist corrosion and wear and at the same time would be available at low production costs.

This study focuses on development of a surface alloying technique for WCB steel castings to impart superior properties on the outer surface. The gravity sand casting technique is explored in this study for in-situ surface alloying of WCB steel to a targeted composition that can be compared to that of stainless-steel alloys currently available in the industry specifically focusing on 316L stainless steel and 2205 duplex stainless steel (DSS). The process developed in this study focuses on improving the WCB steel. The experiments were performed at Maynard Steel, Badger Alloys and the UWM Foundry Lab with samples and prototypes being cast using WCB steel as base metal and Ni, Cr, Fe-Mn, Fe-Si, and Mo being used as the surface alloying elements. REFCOHOL 1010 and REFCOBAR Gel were the binders used in this process for preparing the slurry which was applied to the mold cavity prior to casting for surface alloying of WCB steel. The objective of the study was to improve selected properties of the surface of cast WCB steel components as compared to the base metal. This was done by controlling the composition and quantities of the alloying elements used in the slurry. All samples and prototypes after sectioning showed clear formation of a surface alloyed layer ranging from 100 μm to 4000 μ in depth in all samples made at UWM, Maynard Steel and Badger Alloys. The surface alloyed layer has increased amounts of Ni, Cr and other elements depending up on the slurry applied on the mold prior to casting.

The samples showed an increase in the hardness of the surface alloyed layer as high as 54% as compared to the base metal over the range of the four trials performed at three different locations. Austenite and ferrite were the primary phases identified in the surface alloyed layer using XRD. Pearlite colonies were observed at the interface between the surface alloyed layer and base metal. The formation of σ phase was observed at the grain boundaries in the surface alloyed layer which was rich in Cr. The depth of the surface alloyed layer was controlled in the samples which were cast at the UWM Foundry Lab and the highest depth of surface alloyed layer was 4000 μ.

Industrial trials of the surface alloying process to successfully cast surface alloyed butterfly valve prototypes proves the feasibility of the process on an industrial scale. The heat treatment of the surface alloyed layer by solution annealing at 1000 °C for 10 minutes led to dissolving of the σ phase, diffusion of carbon from the interface layer into the base metal, and promotion of a dual phase in the surface alloyed layer for samples with the targeted composition of 2205 DSS. The process leads to reduction of the overall costs of the components while imparting superior properties as compared to WCB steel.

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