Cast steel

The need for higher efficiencies in combustion engines and power plants means increased operating temperatures and pressures. Consequently, cast steel materials must be adapted to these demanding boundary conditions. Principally, ferritic steels and austenitic steels are capable candidates for this challenge.

Important aspects of a successful material are its creep and oxidation resistance. Due to alloying cost aspects, ferritic grades are preferred where technically possible. The relatively low coefficient of thermal expansion of the ferritic steels as compared to austenitic grades also favors thermal fatigue properties. Molybdenum is an established alloying element in such materials as it improves resistance both to creep and high temperature corrosion.

For large power plant components, 9-10% chromium steels with either a ferritic or martensitic matrix play an important role. Conventional martensitic steels such as X20CrMoV12-1 do not fulfill the increased endurance demand of 100,000 hours at a 600°C operating temperature and 100 MPa stress.

An improved cast steel (G-X12CrMoWVNbN10-1-1) was developed to tolerate an operational condition of 620°C and 100 MPa. The steel contains 1% molybdenum, up to 0.07% niobium and 0.2% vanadium. Niobium and vanadium form mixed MX type carbides whereas molybdenum and tungsten form M₂₃C₆ type carbides and Laves phase together with chromium and iron. These precipitates represent strong barriers against creep at high operating temperatures.

Austenitic cast steel for operation at even higher temperatures contains 18-38% nickel, 15-26% chromium, up to 2.5% silicon and carbon from 0.15 to 0.6%. This chemical composition guarantees a stable austenitic structure at both high and ambient temperatures.

Other alloy additions aim at a simultaneous improvement of the following three parameters: creep resistance, operating temperature, and resistance to aggressive gas environments. In that sense, molybdenum alloying to austenitic irons has similar effects as in ferritic SiMo alloys. The chemical compositions of commonly used stainless steel castings are classified into “C” and “H” series alloys. In the H series, both phase stability and mechanical properties at high operating temperatures are the factors of primary importance. In the C series, the aim is to attain the most corrosion resistant microstructure. Molybdenum is typically added to a level of 0.5 in C series alloys, while selected H series alloys contain molybdenum to much higher levels (e.g., Type 316 stainless steel: 2-3% molybdenum, Type 303: 1.5% molybdenum, Type 317: 3-4% molybdenum).

High-speed steels (HSS) are mainly used as tools for cutting or rolling diverse materials at high speed. During operations, the tool material is exposed to high force, intense friction, and high temperatures. Thus, high yield strength, fracture toughness and wear resistance are essential properties of the tool material. Generally, the matrix of the tool material consists of tempered martensite containing various types of hard carbide particles.

Properties of transition metal carbides
HSS alloys belong to the iron-carbon-X multicomponent system, where X represents a group of alloying elements with chromium, tungsten, molybdenum, vanadium and cobalt being the principal ones. The most important changes caused by progress in alloy design concern the type, morphology, and volume fraction of the eutectic carbides.

Tungsten and molybdenum have nearly identical alloy effects, as reflected from the similarity between iron-tungsten-carbon and iron-molybdenum-carbon ternary phase diagrams. On a mass fraction basis, this can be expressed by a “tungsten equivalent” value defined as molybdenum/tungsten=1:2. However, that similarity is very limited because in the typical composition ranges of high-speed steels, the iron-tungsten-carbon diagram presents only M₆C carbide, while the iron-molybdenum-carbon diagram also presents Mo₂C.

In the composition of M₆C carbides, the letter M corresponds to the elements tungsten, molybdenum, vanadium, chromium, and especially iron. Tungsten and molybdenum are dissolved in the same proportion as represented by the composition of the alloy. Eutectic Mo₂C carbides form with a rod-shaped morphology and are well dispersed in the matrix. However, it is not advisable to substitute all tungsten with molybdenum, as some of the oxidation and corrosion properties might be diminished. The molybdenum level is ideally set at 4.0-8.0%.

Mill rolls for both hot rolling and cold rolling are a main application area of cast HSS steel. Important for such rolls is a high shore hardness and hardening depth as well as sufficient toughness to avoid unexpected roll failure. Any additional means of increasing the wear resistance in the roll surface prolongs the operational endurance, saving downtime due to roll changes and maintenance in the rolling mill. The hardness of the roll sleeve correlates with the volume fraction of carbides in the steel.

Although an increase in carbon and/or chromium content increases the volume fraction of carbide, wear loss is not necessarily improved. However, the addition of suitable amounts of molybdenum effectively improves the wear resistance of the material. Wear is related to cracking of carbides under service condition, especially that of the protuberating MC carbides at the roll surface. The addition of molybdenum reduces cracking susceptibility of carbide particles due to a toughening effect when molybdenum atoms enrich in carbides. This effect maximizes when molybdenum is alloyed in the range of 4-8%.