Flame Hardening of Gray Iron

Flame hardening is the method of surface hardening most commonly applied to gray iron. After flame hardening, a gray iron casting consists of a hard, wear-resistant outer layer of martensite and a core of softer gray iron, which during treatment does not reach the Aj transformation temperature (in fact, the unhardened metal immediately below the hardened case, which has been heated by the flame to some extent, may even be partially annealed during flame hardening if it is unalloyed).

Recommended Composition. Both unalloyed and alloyed gray irons can be successfully flame hardened. However, some compositions yield much better results than do others. One of the most important aspects of composition is the combined carbon content, which should be in the range of 0.50 to 0.70%, although irons with as little as 0.40% combined carbon can be flame hardened. In general, flame hardening is not recommended for irons that contain more than 0.80% combined carbon because such irons (mottled or white irons) may crack in surface hardening.

The stability of the microconstituents from which the carbon precipitates is a factor in determining the hardness of the flame-hardened iron. One user observed that the stability of microconstituents containing combined carbon is indicated by the hardness of test specimens annealed at 845 °C (1550 °F). A test of four pearlitic irons from different sources, containing 3.30% C max, 1.40 to 1.60% Si, 0.80 to 1.10% Mn, and 0.15 to 0.20% Cr, gave the results:

Iron

Hardness, HB

Hardness(a), HRC

As-cast

Annealed

A

220

200

47-50

B

190

180

47-50

C

200

140

35-38

(a) After flame hardening as-cast iron

For maximum hardness, it is advisable to use an iron containing as small an amount of total carbon as is consistent with the production of sound castings free from any danger of cracking. The coarse graphite flakes typical of high-carbon irons should be avoided because they may be burned out during flame heating, thereby producing a porous and unattractive surface.

Because silicon promotes the formation of graphite and of a low combined carbon content, a relatively low silicon content is also advisable. Several users report that silicon content should not exceed 2% in any iron submitted to a flame-hardening operation, although alloy irons containing 2.4% Si can be hardened successfully. They also recommend that manganese content be held in the range of 0.80 to 1.00% to increase carbon solubility in austenite.

Gray iron to be flame hardened should be as free as possible from porosity and from foreign matter such as sand or slag because porosity and even small inclusions of foreign matter can produce a rough surface or result in cracking after hardening. Rough casting surfaces should be sand or shot blasted prior to heat treatment because skin or scale on the surface acts as a heat insulator and reduces the effectiveness of flame hardening.

Effects of Alloying Elements. In general, alloyed gray irons can be flame hardened with greater ease than can unalloyed irons, partly because alloyed gray irons have increased hardenability. Final hardness also may be increased by alloying additions. The maximum hardness obtainable by flame hardening an unalloyed gray iron containing approximately 3% total carbon, 1.7% Si, and 0.60 to 0.80% Mn ranges from 400 to 500 HB. This is because the Brinell hardness value for gray iron is an average of the hardness of the matrix and that of the relatively soft graphite flakes. Actually, the matrix hardness on which wear resistance depends approximates 600 HB. With the addition of 2.5% Ni and 0.5% Cr, an average surface hardness of 550 HB can be obtained. The same result has been achieved using 1.0 to 1.5% Ni and 0.25% Mo. Small additions of chromium are particularly valuable in preventing softening and ensuring the retention of a high content of combined carbon during austenitizing for hardening. Automotive camshafts containing 1% Cr, 0.50% Mo, and 0.8% Mn are easily flame hardened to 52 HRC to a required depth. These parts are not tempered or stress relieved.

Stress Relieving. Whenever practicable or economically feasible, flame-hardened castings should be stress relieved at 150 to 200 °C (300 to 400 °F) in a furnace, in hot oil, or by passing a flame over the hardened surface. Such a treatment will minimize distortion or cracking and will increase the toughness of the hardened layer.

Stress relieving at 150 °C (300 °F) for 7 h was found to remove 25 to 40% of the residual stresses in a flame-hardened casting, while reducing the hardness of the surface by only 2 to 5 points on the HRA scale. Although stress relieving is desirable, it can often be safely omitted.

Hardness. The surface of flame-hardened gray iron typically has a somewhat lower hardness than the metal immediately below the surface (Fig. 18). This decrease in hardness may be caused by the retention of relatively soft austenite at the surface. Surface hardness often can be raised by heating in the range from 195 to 250 °C (380 to 480 °F).

CD I

"P

Distance below surface, 0.001 in. 20 60 100 140

I

1

1

1

1000 2000 3000 Distance below surface, jim

4 GOO

Fig. 18 Typical hardness gradient produced in gray iron by flame hardening

The depth and microstructure of the hardened layer depend on two other factors in addition to temperature: the amount of carbon and alloying elements in solution when the flame-hardened surface is quenched, and the efficiency of quenching. If softness is due to the presence of austenite in the microstructure, subjecting the part to -40 °C (-40 °F) for 1 h will transform the austenite and increase the hardness.

Fatigue strength is usually increased by surface hardening because the treatment induces compressive stresses at the surface. For example, flame or induction heating followed by water quenching induces high compressive stresses (>205 MPa, or 30 ksi) in the fillet areas of crankshafts; however, if self-quenching instead of water quenching is used, undesirable tensile stresses may result at the surface. The degree of improvement in fatigue strength by flame hardening cannot be produced by through hardening.

Quenching. The various methods of flame hardening influence the selection of the quenching medium. In the progressive method, only nonflammable media such as water, soluble-oil mixtures, and solutions of polyvinyl alcohol in water can be employed. Conventional oil cannot be used because of the fire hazard. In spot hardening or spinning methods, in which the flame head is withdrawn from the part before quenching, parts are quenched conventionally by immersion in hot oil.

When quenching is done with water, the water should be at about 30 °C (90 °F) for best results. Lower quenching rates, such as those obtained with 5 to 15% soluble-oil mixtures, compressed air, or compressed air and water at low pressure, are used to prevent cracking. Air quenching is especially suited to highly alloyed cast irons because of their susceptibility to cracking.

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