Malleable irons, both pearlitic and ferritic are obtained after malleablizing-annealing of white cast iron of controlled compositions. As this annealing heat treatment takes long time, moreover there are difficulties in cooling thick sections rapidly enough to get the original white cast iron, S.G. iron has replaced the malleable iron in many applications. However, thin-section components that require maximum machinability and wear-resistance are made of malleable iron.
Hardening and Tempering of Malleable Iron:
Increasing the austenitising temperature and time at that temperature increases the amount of dissolved carbon in austenite (which is also called combined carbon in the matrix after quenching to room temperature) and results in more homogeneous austenite, transforming to more uniform martensite.
But higher temperatures also increase the risk of distortion and cracking. Fig. 15.24 illustrates the effect of austenitising temperature on the hardness of the as-quenched (pearlitic as well as ferritic) malleable irons. The appropriate austenitising temperature for pearlitic malleable iron is 885°C, and for ferritic malleable is 900°C.
The hardening of pearlitic malleable iron consists of following steps:
ADVERTISEMENTS:
1. The castings are air-quenched after first stage graphitisation (FSG), where the massive carbides (of white iron) are eliminated, i.e. from 900°C. These results in retention of about 0.75% combined carbon in the matrix, but may not be homogeneously distributed.
2. Reheating and holding the castings for 1 h at 885°C. It reaustenitizes the matrix, and produces a homogeneous austenite.
3. The castings are quenched in hot (50-55°C) and agitated oil, resulting in martensitic matrix of hardness 555 to 627 BHN (depending on dissolved carbon).
ADVERTISEMENTS:
4. The castings are reheated to be tempered at 600-700°C (depending on the hardness and ductility to be induced), normally, at 650°C for at least 2 h at the temperature to ensure uniformity of the properties (though tempering time is also dependent on the section-thickness and the microstructure of casting). Tempering of martensite is primarily temperature dependent with time being secondary. During tempering, carbide has a tendency to decompose such that carbon deposits on existing temper carbon. This tendency is least at lower tempering temperature, or in suitably alloyed pearlitic malleable iron.
The ferritic malleable iron can be processed to obtain similar microstructures as obtained in pearlitic malleable iron. The ferritic malleable iron is heated to 900-930°C for a long time to get a homogeneous austenite. The combined carbon of the matrix, thus, obtained, is slightly lower than that of pearlitic malleable iron after air-quenching from 900°C. The tempering temperature to get hardness is at a lower level.
Martempering of Malleable Iron:
Martempering substitutes hardening in oil and tempering of pearlitic malleable irons which are susceptible to quench-cracking. The iron after austenitizing at 885°C for 1 h are safely quenched in salt or oil bath maintained at about 200°C. and then air-cooled.
The castings are tempered to get desired ductility and strength. The process results in typical values as- tensile strength 860 MPa; yield strength 760 MPa; hardness, 300 BHN. Elevator camshafts, wear-chain components are made of malleable (pearlitic) iron, which is martempered.
Austempering of Malleable Iron:
ADVERTISEMENTS:
Pearlitic malleable iron can be austempered to get bainite, which results in marked increase in tensile strength, yield strength, hardness, but ductility decreases. Table 15.14 compares austempered properties of bainite with annealed and tempered state of same pearlitic malleable iron (2.6%C, 1.4%Si, 0.5%Mn, 0.11% S).
Surface Hardening of Malleable Iron:
Surface hardening methods, flame as well as induction have been used for small parts to result in a macro-surface-hardness of 55-60 HRC. The depth of surface layer hardened is controlled say in induction hardening by close control of power output, operating frequency, heating time and the alloy composition of the fully pearlitic malleable iron. Laser and electron beam methods can easily harden selected areas of both pearlitic and ferritic malleable iron castings.
Rocker arms and clutch hubs of automobile, crimping tool jaws for electrical connectors are induction hardened to get required wear-resistance. Pinion spacers are flame-hardened to a hardness of 58 HRC to a depth of about 2.4 mm.