Rapid Alloy Surface Engineering through Closed-Vessel Reagent Pyrolysis

Cyprian Illing,Anna Agaponova,Zhe Ren,Arthur Heuer,Frank Ernst

doi:10.3390/met11111764

Abstract

For rapid surface engineering of Cr-containing alloys by low-temperature nitrocarburization, we introduce a process based on pyrolysis of solid reagents, e.g., urea, performed in an evacuated closed vessel. Upon heating to temperatures high enough for rapid diffusion of interstitial solute, but low enough to avoid second-phase precipitation, the reagent is pyrolyzed to a gas atmosphere containing molecules that (i) activate the alloy surface by stripping away the passivating Cr2O3-rich surface film (diffusion barrier) and (ii) rapidly infuse carbon and nitrogen into the alloy. We demonstrate quantitatively that this method can generate a subsurface zone with concentrated carbon and nitrogen comparable to what can be accomplished by established (e.g., gas-phase- or plasma-based) methods, but with significantly reduced processing time. As another important difference to established gas-phase processing, the interaction of gas molecules with the alloy surface can have auto-catalytic effects by altering the gas composition in a way that accelerates solute infusion by providing a high activity of HNCO. The new method lends itself to rapid experimentation with a minimum of laboratory equipment.

Highlights

The mechanical behavior and the corrosion resistance of a broad spectrum of Crcontaining structural alloys can be substantially improved by infusing concentrated interstitially dissolved carbon or nitrogen through the alloy surface at low temperature [1,2,3,4,5,6,7,8,9]
We demonstrate the efficacy and potential of this method for exploiting and exploring pyrolysis of solid reagents for the purpose of alloy surface engineering by low-temperature infusion of carbon and nitrogen
On the right side of h[z], at a depth h ≥ 35 μm, to which no significant amounts of interstitial solute could diffuse within the processing time, the diagram displays a hardness range of h = (350 ± 50) HV0.025

Summary

Introduction

The mechanical behavior and the corrosion resistance of a broad spectrum of Crcontaining structural alloys can be substantially improved by infusing concentrated interstitially dissolved carbon or nitrogen through the alloy surface at low temperature [1,2,3,4,5,6,7,8,9]. From a technical point of view, it is desirable to (i) maximize the fraction X of interstitial solute at each depth z below the surface (while avoiding precipitation), (ii) minimize the processing time τp, and (iii) maximize the mean depth z of the interstitial solute below the alloy surface. In the literature, the latter is often loosely characterized as “case depth” or “thickness” ζ of the infused layer, which is physically incorrect because a diffusion profile does not have a sharp depth. To design a process with high efficiency, from a fundamental point of view, there are three distinct components to consider: 1. The chemical potential of solute at the alloy surface

Results

Discussion

Conclusion