Low-Temperature Carburization of AL-6XN Enabled by Provisional Passivation

Zhen Li,Frank Ernst,Cyprian Illing,Arthur Heuer

doi:10.3390/met8120997

Abstract

Employing AISI-AL-6XN as example, we introduce a new method of surface activation for low-temperature carburization. This method consists of two steps: (i) removing the passivating surface oxide and a potentially existing severely plastically deformed surface layer (Beilby layer) by aqueous (liquid) hydrochloric acid, and (ii) immersion in ethanol and subsequent drying in nitrogen. Upon carburization with a gas mixture of acetylene, hydrogen, and nitrogen, this new method of surface activation enables the formation of a fully developed “case”, a uniform solid solution of interstitial carbon in austenite with carbon fractions up to 0.20 near the alloy surface. The underlying mechanism of surface activation is shown to involve the formation of a provisional passivating layer. It consists of chlorides or ethoxides that are insoluble in ethanol. It prevents the reformation of the regular Cr-rich passivating oxide layer and is readily removed upon heating and exposure to the carburizing gas. As the new activation method is quicker, more effective, and less destructive to furnace hardware than activation with hot gaseous hydrochloric acid that is currently applied in industrial manufacturing, it may have considerable technological impact.

Highlights

Low-temperature carburization is a potent method of surface engineering
We report the discovery that the industrially established in situ surface activation process using gaseous HCl can effectively be replaced by the following ex situ procedure: (i) immersing the alloy part in a liquid etchant at room temperature, (ii) stopping the etch process by directly transferring the specimen into a reservoir of C2 H5 OH, (iii) loading the specimens into a furnace for gas-phase low-temperature carburization, purged with dry N2 gas, (iv) and heating them and exposing them to streaming carburizing gas
The left column contains the micrographs from specimens that were activated by etching in aqueous HCl, whereas the right column shows micrographs from specimens prepared without activation

Summary

Introduction

It can be applied to alloys that contain significant fractions of alloying elements with a high affinity for carbon. The prototype of such an alloy is the austenitic stainless steel AISI-316L (UNS designation S31603) with an atom fraction XCr = 0.18 of Cr as the high-carbon-affinity, carbide-forming element. Owing to the high stability of Cr carbides, the equilibrium-solubility limit of carbon in this alloy is very small, corresponding to a carbon atom fraction ◦XC ≈ 10−6 at 300 K. Even at a high temperature, ◦XC is significantly smaller than the technically desired levels of e.g., XC ≈ 0.1.

Methods

Results

Conclusion