Influence of Thermal Treatment on Microstructure and Hardness of Niobium Alloyed PM‐Tool Steel

Abstract

This work describes carbides present in the tool steel containing 2.5% C, 3.3% Si, 6.2% Cr, 2.2% Mo, 2.6% V, 2.6% Nb, 1.0% W, prepared by the powder metallurgy (PM) process. The influence of thermal treatment conditions on carbide behavior is investigated. During austenitization, some dissolution of M 7 C 3 occurs, while MC carbides are thermally stable. So, the quenched steel is composed of two types of carbides: chromium rich M 7 C 3 and niobium rich MC, besides martensite and retained austenite. During tempering of the hardened steel at the temperatures above 4808C, the precipitation of very fine carbides and martensite recovery occur, which results in secondary hardening. Precipitated particles are predominantly vanadium and molybdenum rich MC, M 2 C, and M 6 C carbides.

Processing of Niobium-Alloyed High-Carbon Tool Steel via Additive Manufacturing and Modern Powder Metallurgy

“…The microstructure of the spark plasma sintered material in the as-sintered state is composed of various types of carbides embedded in an iron-based matrix (see Figure 2 ). The almost rounded particles, which appear darker than the matrix in the SEM-BSE image, are chromium-rich M 7 C 3 carbides [ 24 ]. The coarse light ones contain niobium and molybdenum ( Figure 3 ), while the fine ones are composed of niobium, molybdenum and vanadium.…”

“…The coarse light ones contain niobium and molybdenum ( Figure 3 ), while the fine ones are composed of niobium, molybdenum and vanadium. These carbides are, hence, of MC type [ 24 ]. Their chemical composition is presented below in Table 2 .…”

“…Their chemical composition is presented below in Table 2 . The above assignment of the phases is based on experience with determining their microstructure using a combination of chemical analysis and SAED in a previous study using a material processed by HIP, which was published recently [ 24 ].…”

Solutions of Critical Raw Materials Issues Regarding Iron-Based Alloys

“…Due to limited thermal stability, these alloys could be used just as cold-work tool steels. At the beginning of the 21st century, there were also trials in substituting vanadium partially with niobium [ 63 , 64 , 65 , 66 ]. These elements form the same type of carbide (MC), while NbC is harder than VC.…”

“…On the other hand, niobium carbide tends to coarsen during metallurgical processing. Even though a suitable manufacturing route (rapid solidification powder metallurgy) [ 66 ] and appropriate surface treatment [ 67 ] were developed, it did not lead to a high success due to the increasing use of niobium and its low availability, which led to its listing as a CRM. The other possibility for substituting vanadium partially is titanium [ 63 ].…”