Diffusion Methods

As previously mentioned, surface hardening by diffusion involves the chemical modification of a surface. The basic process used is thermochemical because some heat is needed to enhance the diffusion of hardening species into the surface and subsurface regions of a part.

Methods of hardening by diffusion include several variations of hardening species (such as carbon or nitrogen) and of the process method used to handle and transport the hardening species to the surface of the part. Process methods for exposure involve the handling of hardening species in forms such as gas, liquid, or ions. These process variations naturally produce differences in typical case depth and hardness. Table 39 compares the characteristics of various diffusion treatments.

Table 39 Typical characteristics of diffusion treatments

Process

Nature of case

Process temperature, °C ( °F)

Case hardness, Typical case depth

HRC

Typical base metals

Process characteristics

Carburizing

Pack

Diffused carbon

815-1090

125 ^m-1.5 mm (5-60

50-

Low-carbon steels, low-

Low equipment costs, difficult to

(1500-2000)

mils)

63(a)

carbon alloy steels

control case depth accurately

Gas

Diffused carbon

mm (3-60 mils)

63(a)

Low-carbon steels, low-carbon alloy steels

Good control of case depth, suitable for continuous operation, good gas controls required, can be dangerous

Liquid

Diffused carbon and possibly nitrogen

815-980 (1500-1800)

50 ^m-1.5 mm (2-60 mils)

65(a)

Low-carbon steels, low-carbon alloy steels

Faster than pack and gas processes, can pose salt disposal problem, salt baths require frequent maintenance

Vacuum

Diffused carbon

mm (3-60 mils)

63(a)

Low-carbon steels, low-carbon alloy steels

Excellent process control, bright parts, faster than gas carburizing, high equipment costs

Nitriding

Gas

Diffused nitrogen, nitrogen compounds

480-590 (9001100)

125 ^m-0.75 mm (5-30 mils)

50-70

Alloy steels, nitriding steels, stainless steels

Hardest cases from nitriding steels, quenching not required, low distortion, process is slow, is usually a batch process

Salt

Diffused nitrogen, nitrogen compounds

510-565 (9501050)

2.5 ^m-0.75 mm (0.1-30 mils)

50-70

Most ferrous metals including cast irons

Usually used for thin hard cases <25 ^m (1 mil), no white layer, most are proprietary processes

Ion

Diffused nitrogen, nitrogen compounds

340-565 (6501050)

75 ^m-0.75 mm (3-30 mils)

50-70

Alloy steels, nitriding steels, stainless steels

Faster than gas nitriding, no white layer, high equipment costs, close case control

Carbonitriding

Gas

Diffused carbon and nitrogen

760-870 (1400-1600)

75 ^m-0.75 mm (3-30 mils)

65(a)

Low-carbon steels, low-carbon alloy steels, stainless steel

Lower temperature than carburizing (less distortion), slightly harder case than carburizing, gas control critical

Liquid (cyaniding)

Diffused carbon and nitrogen

760-870 (1400-1600)

2.5-125 ^m (0.1-5 mils)

65(a)

Low-carbon steels

Good for thin cases on noncritical parts, batch process, salt disposal problems

Ferritic nitrocarburizing

Diffused carbon and nitrogen

480-590 (9001090)

2.5-25 ^m (0.1-1 mil)

60(a)

Low-carbon steels

Low-distortion process for thin case on low-carbon steel, most processes are proprietary

Source: Ref 64

(a) Requires quench from austenitizing temperature.

Carburizing is the addition of carbon to the surface of low-carbon steels at temperatures (generally between 850 and 950 °C, or 1560 and 1740 °F) at which austenite, with its high solubility for carbon, is the stable crystal structure. Hardening is accomplished when the high-carbon surface layer is quenched to form martensite so that a high-carbon martensite case with good wear and fatigue resistance is superimposed on a tough, low-carbon steel core. Carburizing steels for case hardening usually have base-carbon contents of about 0.2%, with the carbon content of the carburized layer generally being controlled at between 0.8 and 1% C.

Carburizing methods include:

• Gas carburizing

• Vacuum carburizing

• Plasma carburizing

• Salt bath carburizing

• Pack carburizing

These methods introduce carbon by the use of gas (atmospheric-gas, plasma, and vacuum carburizing), liquids (salt bath carburizing), or solid compounds (pack carburizing). All of these methods have advantages and limitations, but gas carburizing is used most often for large-scale production because it can be accurately controlled and involves a minimum of special handling.

Nitriding is a surface-hardening heat treatment that introduces nitrogen into the surface of steel at a temperature range of 500 to 550 °C (930 to 1020 °F) while it is in the ferritic condition. Thus, nitriding is similar to carburizing in that surface composition is altered but different in that nitrogen is added into ferrite instead of austenite. Because nitriding does not involve heating into the austenite phase field and a subsequent quench to form martensite, nitriding can be accomplished with a minimum of distortion and with excellent dimensional control. Process methods for nitriding include gas, liquid (salt bath), and plasma (ion) nitriding.

Nitrided steels are generally medium-carbon (quenched and tempered) steels that contain strong nitride-forming elements such as aluminum, chromium, vanadium, and molybdenum. The most significant hardening is achieved with a class of alloy steels (nitralloy type) that contain about 1% Al. When these steels are nitrided, the aluminum forms AlN particles, which strain the ferrite lattice and create strengthening dislocations. Titanium and chromium are also used to enhance case hardness, although case depth decreases as alloy content increases. The microstructure also influences nitridability because ferrite favors the diffusion of nitrogen and because a low carbide content favors both diffusion and case hardness. Usually alloy steels in the heat-treated (quenched and tempered) state are used for nitriding.

Carbonitriding is a surface-hardening heat treatment that introduces carbon and nitrogen in the austenite of steel. This treatment is similar to carburizing in that the austenite composition is changed and high surface hardness is produced by quenching to form martensite. However, because nitrogen enhances hardenability, carbonitriding makes possible the use of low-carbon steel to achieve surface hardness equivalent to that of high-alloy carburized steel without the need for drastic quenching, resulting in less distortion and reducing the danger of cracking the work. To some extent, hardening is also dependent on nitride formation.

Although the process of carbonitriding can be performed with gas atmospheres or salt baths, the term carbonitriding often refers solely to treatment in a gas atmosphere. Basically, carbonitriding in a salt bath is the same as cyanide bath hardening. In both processes, nitrogen enhances hardenability and case hardness but inhibits the diffusion of carbon.

Ferritic nitrocarburizing involves the diffusion of carbon and nitrogen into the ferrite phase and the formation of a thin white layer of carbonitrides. The diffusion of nitrogen into the substrate is necessary for fatigue resistance. The case depths are thin (Table 39), but have a reduced tendency to spall, compared to the white layer formed during conventional nitriding. Ferritic nitrocarburizing, which is used to improve the anti-skuffing properties of steels, can be carried out by gas or plasma (ion) methods.

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