Heat Treating of Malleable Irons


Ferritic and pearlitic malleable irons are both produced by annealing white iron of controlled composition. Malleable irons have largely been replaced by ductile iron in many applications. This is due in part to the necessity of lengthy heat treatments for malleable iron and the difficulty in cooling thick sections rapidly enough to produce white iron. Malleable iron is still often preferred for thin section castings and parts that require maximum machinability and wear resistance.

Ferritic and pearlitic malleable irons are both produced by annealing white iron of controlled composition. Malleable irons have largely been replaced by ductile iron in many applications. This is due in part to the necessity of lengthy heat treatments for malleable iron and the difficulty in cooling thick sections rapidly enough to produce white iron. Malleable iron is still often preferred for thin section castings and parts that require maximum machinability and wear resistance.

The annealing of malleable iron should be done in a furnace with a controlled atmosphere of dry nitrogen, hydrogen (1.5%), and carbon monoxide (1.5%). The dew point of this mixture should be between -40 and -70°C (-40 and -20°F). These conditions eliminate the possibility of decarburization and loss of learner carbon nodules below the casting surface.

The annealing treatment involves three important steps:

  • The first causes nucleation of temper carbon. It is initiated during heating to a high holding temperature and occurs very early during the holding period.
  • The second step consists of holding at 900 to 970°C (1650 to 1780°F); this step is called first-stage graphitization (FSG). During FSG, massive carbides are eliminated from the iron structure. Long holding periods at 955°C (1750°F) will reduce the solubility of nitrogen in iron (which should be kept at 80 to 120 ppm), thereby reducing the mechanical properties of the iron. This occurrence should be kept in mind for long, or "weekend" holding periods. When the carbides are eliminated, the iron is rapidly cooled to 740°C.
  • The third step in the annealing treatment consists of slow cooling through the allotropic transformation range of the iron; this step is called second-stage graphitization (SSG). During SSG a completely ferrule matrix free of pearlite and carbides is obtained when the cooling rate is 2 to 28°C/h (3 to 50°F/h). This cooling rate, which depends on the silicon content of the iron and the temper carbon nodule count, may be increased to 85°C/min (150°F/min) by air quenching from 900°C (1650°F) to form a pearlitic matrix. Oil quenching from 900°C (1650°F) will produce a martensitic matrix. However, unless the temperature in the furnace is lowered to 845°C (1550°F) for at least 4 h (plus 1 h for each 25 mm or inch of section casting thickness), prior to uniform quenching in oil, the matrix microstructure will not be uniform in combined carbon. This nonuniformity reduces machinability. If the hardness is reduced by extended tempering, the resulting structure may not have a good response to selective hardening.

Hardening and Tempering of Malleable Iron

A typical procedure for producing a hardened pearlitic malleable iron consists of:
  • first air quenching castings after first-stage annealing, which results in retention of about 0.75% combined carbon in the matrix;
  • second, reheating and holding for 1 hour at 885°C (l625°F) to reaustenitize the matrix and homogenize the combined carbon; and then
  • quenching in heated and agitated oil, thereby developing a matrix consisting of martensite without bainite and having a hardness of 555 to 627 HB.
The appropriate austenitizing temperature for pearlitic malleable iron is 885°C (1625°F) and for ferritic malleable iron it is 900°C (1650°F). If direct oil quenching is used, caution must be exercised to prevent cracking due to high combined carbon.

Air-quenched and pearlitic malleable iron has a matrix consisting of a ferrite ring around the temper carbon (which produces a lower yield strength) and partially broken lamely pearlite. The remaining lamellar pearlite reduces machinability to a limit of 240 HB.

Increasing the austenitizing time and temperature increases the amount dissolved carbon, which is measured as combined carbon in the matrix after quenched to room temperature. Austenitizing temperatures in the range of 900 to 930°C (1650 to 1700°F) result in a more homogeneous austenite, which is desirable for more uniform martensite. Higher temperatures can result in a greater tendency toward distortion or cracking. Tempering of pearlite is time and temperature dependent. Tempering of martensite is primarily temperature dependent, while time being secondary.

Hardened and tempered pearlitic malleable iron can also be produced from fully annealed ferritic malleable iron, the matrix of which is essentially carbon-free: graphite can be dissolved in austenite by holding at 900 to 930°C (1650 to 1700°F) for a time sufficiently long for the production of an austenite matrix of uniform carbon content. In general, the combined carbon content of the matrix produced by this procedure is slightly lower than they of a pearlitic malleable iron made by air quenching directly from 900°C (1650°F).

Tempering treatments consist of cycles of no less than 2 h at temperature to ensure uniformity of product. Tempering times must also be adjusted for section thickness and quenched microstructures. Fine pearlite and bainite require longer tempering times than that for martensite. In general, final hardness is controlled with process controls approximately the same as those encountered in the heat treatment of medium-carbon and higher-carbon steels. This is particularly true when the specification requires final hardnesses in the range from 241 to 321 HB.

The effects of tempering on the hardness of alloyed and unalloyed malleable irons illustrate the beneficial effects of alloying on as-quenched hardness and stability at elevated temperatures. During all tempering treatments, carbide has a tendency to decompose, with resulting deposition of graphite on existing temper carbon nodules. This tendency is least at the lower tempering temperatures or in suitably alloyed pearlitic malleable irons.

Martempering and tempering develops mechanical properties similar to those resulting from conventional oil quenching and tempering: typical tensile strength 860 MPa (125 ksi), yield strength 760 MPa (110 ksi), and hardness 300 HB.

Pearlitic malleable iron castings that are susceptible to cracking when quenched in warm oil (40 to 95°C, or 100 to 200°F) from the austenitizing temperature may be safely quenched in salt or oil at about 200°C (400°F). Elevator camshafts varying in length from 0.3 to 0.45 m (12 to 18 in.) and various sizes of wear-chain components are examples of martempered pearlitic malleable iron.

Bainitic Heat Treatment of Pearlitic Malleable Iron

Both upper and lower bainite can be formed in pearlitic malleable iron with a marked increase in tensile strength and hardness but with a decrease in ductility.

A pearlitic malleable iron (2.6C-1.4Si-0.5Mn-0.1S), annealed at 930°C (1700°F) for 16 h, air quenched and tempered at 680°C (1250°F) for 4 h, developed an ultimate tensile strength of 650 MPa (94.2 ksi), a yield strength of 460 MPa (66.5 ksi), and a 3.4% elongation at 217 HB.

This same iron austenitized at 900°C (1650°F) in molten salt for 1 h, quenched in molten salt at 295°C (560°F) for 3 h, and air cooled gave an ultimate strength of 995 MPa (144.2 ksi), a yield strength of 920 MPa (133.4 ksi), and 388 HB.

Surface Hardening of Pearlitic Malleable Iron

Fully pearlitic malleable iron may be surface hardened by either induction heating and quenching or flame heating and quenching. Laser and electron beam techniques also have been used for hardening selected areas on the surface of pearlitic and ferritic malleable iron castings that are free from decarburization.

Generally, hardness in the range from 55 to 60 HRC is attainable, with the depth of penetration being controlled by the rate of heating and by the temperature developed at the surface of the part being hardened. In induction hardening, this is accomplished by the close regulation of power output, operating frequency, heating time, and alloy content of the iron.

The maximum hardness obtainable in the matrix of a properly hardened part is 67 HRc; however, conventional hardness measurements show less than the true matrix hardness because of the temper carbon nodules that are averaged into the hardness. Generally, a casting with a matrix microhardness of 67 HRc will have average hardness of about 62 HRc, as measured with the standard Rockwell tester.

Rocker arms and clutch hubs are examples of automotive production parts that are surface hardened by induction. Flame hardening requires close control for these applications in order to avoid distortion that would interfere with their operation. The two examples that follow describe the successful application of induction and flame hardening to other production parts.

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