Mechanically alloyed (MA) steels

9.2.6 Mechanically alloyed (MA) steels

For strengthening at high temperatures, dispersion strengthening with oxide, nitride or carbide particles is an attractive possibility. Such dispersion-strengthened materials are usually produced by powder processing,

302 Modern Physical Metallurgy and Materials Engineering

Figure 9.5 Effect of second phase particles size d at constant volume fraction f on (a) work-hardening rate, (b) elongation and (c) tensile strength (after Balliger and Gladman, 1981) .

steel and Fe– 25Cr– 6Al – 2Y. However, the most highly developed material is the 20%Cr, 4.5%Al ferritic stainless steel, dispersion-strengthened with

0.5% Y 2 O 3 (MA 956 ). MA 956 which has been made into various fabricated forms has extremely

good high-temperature strength (0.2% proof strength is 200 MN m at 600 °

C, 100 MN m at 1000 °

C and

75 MN m at 1200 ° C). The high-strength capability is combined with exce- ptional high-temperature oxidation and corrosion resis- tance, associated with the formation of an aluminium oxide scale which is an excellent barrier to carbon. No carburization occurs in hydrogen – methane mixtures at 1000 °

C. Sulphidation resistance is also good. Figure 9.6 Bauschinger tests for a 0.06%C, 1.5%Mn,

MA 956 was originally developed for use in sheet 0.85%Si dual-phase steel (courtesy of D. V. Wilson) .

form in gas-turbine combustors but, with its combi- nation of high strength up to 1300 °

C, corrosion resis-

a special form of which is known as mechanical alloy- tance and formability, the alloy has found many other ing (MA).

applications in power stations, including oil and coal Mechanical alloying is a dry powder, high-energy

burners and swirlers, and fabricated tube assemblies ball-milling process in which the particles of elemen-

for fluid-bed combustion.

tal or pre-alloyed powder are continuously welded together and broken apart until a homogeneous mix-

9.2.7 Designation of steels

ture of the matrix material and dispersoid is pro- The original system for labelling wrought steels was duced. Mechanical alloying is not simply mixing on

devised in 1941 and used En numbers. This system

a fine scale but one in which true alloying occurs. was replaced in 1976 by the British Standard (BS) The final product is then consolidated by a combina-

designation of steels which uses a six-unit system. tion of high temperature and pressure (i.e. extrusion of

Essentially, it enables the code to express composi- canned powder) or hot isostatic pressing (i.e. HIPing).

tion, steel type and supply requirements. The latter Further processing is by thermo-mechanical process-

is shown by three letters: M means supply to spec- ing (TMP) to produce either (1) fine equiaxed grains

ified mechanical properties, H supply to hardenabil- for good room-temperature strength and good fatigue

ity requirements and A supply to chemical analysis strength or (2) coarser, elongated grains to give good

requirements. For convenience, steels are divided high-temperature stress – rupture strength and thermal-

into types; namely, carbon and carbon – manganese fatigue resistance.

steels, free-cutting steels, high-alloy steels and alloy Various types of ferrous alloy have been made

steels. For example, carbon and carbon – manganese by mechanical alloying, including 17%Cr, 7%Ni,

steels are designated by mean of Mn/letter/mean of 1.2%Al precipitation-hardened austenitic martensitic

C. Thus 080H41 signifies 0.6 – 1.0 Mn/hardenability

Modern alloy developments 303 requirement/0.38 – 0.45 C. Free-cutting steels are des-

At the low carbon contents of typical steels, graphite ignated by 200 – 240/letter/mean of C. Thus 225M44

is not formed, however, because of the sluggishness signifies free-cutting 0.2 – 0.3 S/mechanical properties

of the reaction to graphite. But when the carbon con- requirement/0.4 – 0.48 C with 1.3 – 1.7 Mn. High-alloy

tent is increased to that typical of cast irons (2 – 4% C) steels include stainless and valve steels. The desig-

either graphite or cementite may separate depending nation is similar to the AISI system and is given by

on the cooling rate, chemical (alloy) composition and 300 – 499/letters/variants 11 – 19. Thus 304S15 (pre-

heat treatment (see Figure 9.7). When the carbon exists viously known as Type 304 as used by the AISI)

as cementite, the cast irons are referred to as white signifies 0.06 max. C, 8 – 11 Ni, 17.5 – 19 Cr. Alloy

because of the bright fracture produced by this brit- steels are designated by 500 – 999/letter/mean of C.

tle constituent. In grey cast irons the carbon exists Thus 500 – 519 are Ni steels, 520 – 539 Cr steels,

as flakes of graphite embedded in the ferrite– pearlite 630 – 659 Ni – Cr steels, 700 – 729 Cr– Mo steels and

matrix and these impart a dull grey appearance to the 800 – 839 Ni – Cr– Mo steels. Typically 530M40 signi-

fracture. When both cementite and graphite are present fies 0.36 – 0.44 C, 0.9 – 1.2 Cr, supplied to mechanical

a ‘mottled’ iron is produced. properties.

High cooling rates, which tend to stabilize the Tables 9.1 and 9.2 give the compositions of typical

carbon, alloy and stainless steels. cementite, and the presence of carbide-formers give rise to white irons. The addition of graphite-forming elements (Si, Ni) produces grey irons, even when