Carburizing Boriding and Flame Hardening

Carburizing is a case-hardening process in which carbon is dissolved in the surface layers of a steel part at a temperature sufficient to render the material austenitic, followed by quenching and tempering to form a martensitic microstructure. The resulting gradient in carbon content below the surface of the part causes a gradient in hardness, producing a strong, wear-resistant surface layer. In gas carburizing, commercially the most important variant of carburizing, the source of carbon is a carbon-rich furnace atmosphere produced either from gaseous hydrocarbons (e.g., methane, propane, or butane) or from vaporized hydrocarbon liquids.

Stainless steels can be carburized to improve surface hardness and resistance to galling. The availability of substantial amounts of chromium promotes the rapid formation of chromium carbides at the surface, and surface hardness values of 700 to 750 HV have been measured. Precipitation-hardenable stainless steels can be gas carburized to improve resistance to galling. Treatment temperatures up to 1010 °C (1850 °F) may be required (Ref 14).

In austenitic grades, much of the chromium carbide formed migrates and is precipitated in the grain boundaries, promoting susceptibility to intergranular corrosion. Carbide distribution tends to be less segregated in the martensitic grades, but they are also subject to localized corrosion and particularly SCC (Ref 14).

Carburizing is not generally recommended for stainless steels because of the reduction in corrosion resistance brought on by chromium precipitation. Since the precipitated chromium tends to exist at the expense of adjacent areas, localized galvanic cells may develop. In the carburizing process, surface physical properties are obtained at the expense of surface chemical properties (Ref 14).

Because of these disadvantages, most of the carburizing of stainless steel is "accidental" and can be traced to surface contamination. Carbonaceous compounds such as charcoal, coke, oil, and grease can promote carburization under the proper conditions. Molten salt baths that contain cyanide to prevent decarburization may also become carburizing in contact with stainless steels (Ref 14).

Boriding, or boronizing, is a thermochemical surface hardening process that involves heating well-cleaned material in the range of 700 to 1000 °C (1300 to 1830 °F), preferably for 1 to 12 h, in contact with a boronaceous solid powder (boronizing compound), paste, liquid, or gaseous medium.

The resultant diffusion zone is known for its low coefficient of friction and high surface hardness, which may reach values of 1800 HV. Case depths of 0.05 mm (0.002 in.) can be achieved with 1 to 5 h treating time (Ref 14). Figure 9 shows the influence of steel composition on abrasive wear resistance. Boriding can considerably enhance the corrosion-erosion resistance of ferrous materials in nonoxidizing dilute acids (Fig. 10) and alkali media, and is increasingly used to this advantage in many industrial applications (Ref 17).

Fig. 9 Effect of steel composition (nominal values in wt%) on wear resistance under abrasive wear (dv = thickness of the boride layer). Test conditions: DP-U grinding tester, SiC paper 220, testing time 6 min. Source: Ref 15, 16

Fig. 10 Corroding effect of mineral acids on boronized and nonboronized (a) 0.45% C (Ck 45) steel and (b) 18Cr-9Ni (X10CrNiTi189) steel at 56 °C (130 °F). Source: Ref 17, 18

Chromium considerably modifies the structure and properties of iron borides. As the chromium content in the base material increases, progressive improvements in the following effects are observed: formation of boron-rich reaction products, decrease in boride depth, and flattening or smoothening of the coating/substrate interface (Ref 19). A reduction of boride thickness has also been noticed in ternary Fe-12Cr-C steels with increasing carbon content (Ref 20).

Flame hardening is a heat treating process in which a thin surface shell of a steel part is heated rapidly to a temperature above the critical point of the steel. After the grain structure of the shell has become austenitic (austenitized), the part is quickly quenched, transforming the austenite directly to martensite while leaving the core of the part in its original state.

Flame hardening employs direct impingement of a high-temperature flame or high-velocity combustion product gases. The part is then cooled at a rate that will produce the desired levels of hardness and other properties. The high-temperature flame is obtained by combustion of a mixture of fuel gas with oxygen or air; flame heads are used for burning the mixture. Depths of hardening from about 0.8 to 6.4 mm (^ to 4in.) or more can be obtained, depending on the fuels used, the design of the flame head, the duration of heating, the hardenability of the work material, and the quenching medium and method of quenching used.

Hardening by flame differs from true case hardening because the hardenability necessary to attain high levels of hardness is already contained in the steel, and hardening is obtained by localized heating. Although flame hardening is mainly used to develop high levels of hardness for wear resistance, the process also improves bending and torsional strength and fatigue life.

Flame hardening can be applied to martensitic stainless steels. The nature of flame hardening, however--especially the relatively high temperature gradients and higher than normal surface temperatures-- may cause the retention of excessive amounts of austenite in many highly alloyed materials, with possible low hardness and transformation to untempered martensite in service, accompanied by brittleness. Typical hardnesses obtained for martensitic stainless steels by flame heating and quenching in air or oil are given in Table 13.

Table 13 Response of martensitic stainless steels to flame hardening

Material

Typical hardness, HRC, as affected by quenchant

Air(a)

Oil(b)

410, 416

41-44

41-44

414, 431

42-47

42-47

420

49-56

49-56

(a) To obtain the hardness results indicated, those areas not directly heated must be kept relatively cool during the heating process.

(b) Thin sections are susceptible to cracking when quenched with oil.

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