Heat Treatment Coatings

Pack Difffised carbon 815-1090 (1500-2000) 125 pjn-1.5 mm (5-60 mils) 50-63(a) Lx w-carbon steels, low-carbon alloy steels Low equipment costs, difficult to control case depth accurately 

Liquid Diffused carbon and possibly nitrogen 815-980 (1500-1800) 50 pjn-1.5 mm (2-60 mils) 50-65(a) Low-carbon steels, low-carbon alloy steels Faster than pack and gas processes, can pose salt disposal problem, salt baths requite frequent maintenance 

Gas Diffused nitrogen, nitrogen compounds 480-590 (900-1100) 125 xm-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 low, is usually a batch process 

---

Today, a customer specifically selects the desired material properties, such as corrosion resistance, weidabiiity, and iow warpage, according to the type of further heat treatment, coating, or etching needed. Property combinations such as good corrosion resistance, good machinabiiity, and much more can therefore be realized. 

The stabihty of a particular treatment is also important. The enhancement should survive during normal wear or display conditions. Whereas all the enhancements from heat treatments are stable, some produced by irradiation are not. There are also surface coatings which wear off, oilings which dry out, etc. 

Selective Carburi ng. In most components, it is desirable to carburize only parts of the surface. To prevent other regions from carburizing, they must be protected. For holes, simple plugs of copper may be used. In some cases, copper plating can be appHed, but diffusion into the steel must be considered, and the copper may have to be machined off later. Coatings (qv), which can be appHed as a paste and then removed after heat treatment, are also available and include copper plating, ceramic coatings, and copper and tin pastes. 

Anhydrous monocalcium phosphate, Ca(H2PObe made in a pan mixer from concentrated phosphoric acid and lime. The high heat of reaction furnishes essentially all the necessary thermal input and subsequent drying is minimized. A small amount of aluminum phosphate or a mixture of sodium and potassium phosphates is added in the form of proprietary stabilizers for coating the particles. Heat treatment converts the coating to a protective polyphosphate (19). 

A principal commercial appHcation of the hypophosphites is ia the electroless plating (qv) process. Nickel salts are chemically reduced by hypophosphites to form a smooth adherent nickel plating to protect the iateriors of large vessels and tank cars. The coating, which can be hardened by heat treatment, usually contains 8—10 wt % phosphoms and is highly impervious. 

In metallurgical practice, sodium uses include preparation of powdered metals removal of antimony, tin, and sulfur from lead modification of the stmcture of siHcon—aluminum alloys appHcation of diffusion alloy coatings to substrate metals (162,163) cleaning and desulfurizing alloy steels via NaH (164) nodularization of graphite in cast iron deoxidation of molten metals heat treatment and the coating of steel using aluminum or zinc. 

Solution Deposition of Thin Films. Chemical methods of preparation may also be used for the fabrication of ceramic thin films (qv). MetaHo-organic precursors, notably metal alkoxides (see Alkoxides, metal) and metal carboxylates, are most frequently used for film preparation by sol-gel or metallo-organic decomposition (MOD) solution deposition processes (see Sol-GEL technology). These methods involve dissolution of the precursors in a mutual solvent control of solution characteristics such as viscosity and concentration, film deposition by spin-casting or dip-coating, and heat treatment to remove volatile organic species and induce crystaHhation of the as-deposited amorphous film into the desired stmcture. 

Practical considerations enter into the experimental plan in various other ways. In many programs, variables are introduced at different operational levels. For example, in evaluating the effect of alloy composition, oven temperature, and varnish coat on tensile strength, it may be convenient to make a number of master alloys with each composition, spHt the alloys into separate parts to be subjected to different heat treatments, and then cut the treated samples into subsamples to which different coatings are appHed. Tensile strength measurements are then obtained on all coated subsamples. 

Some liquid metals have been observed to cause embrittlement in many titanium alloys. In mercury, for example, Ti-8Al-lMo-lV exhibits both intergranular and transgranular fracture with velocities as high as lOcm/s. Heat treatment affects this behaviour in a manner similar to that observed in aqueous and methanolic solutions. Some alloys are embrittled by liquid cadmium and zinc. More surprising, perhaps, is the observed solid metal embrittlement which has been found on titanium alloy components coated with cadmium, silver or zinc Service failures of cadmium-plated Ti-6A1-4V fasteners have been reported , and cracking of this alloy and... 

It must always be remembered that diffusion coatings are produced by a form of heat treatment and that, with the exception of low-temperature zinc diffusion (sherardising), the treated ferrous materials are usually in the annealed condition. Whenever the mechanical properties of the parts must be restored to their original level, a subsequent heat treatment is necessary . This does not as a rule present any difficulty with chromised or boronised steels. In order to prevent undue distortion and internal stresses during treatment and subsequent hardening, it is recommended that high-carbon and alloy steels should be processed in the normalised condition. 

Spray-aluminised coatings are used for exhaust valves in automobile engines, exhaust and silencer systems (double and triple life), tyre moulds, gas ducting, heat-treatment pots, furnace ladles, carburising boxes and fans handling hot gases. Similar applications utilise Calorised and hot-dip aluminised coatings. Hot-dip aluminised steel wire has been used in steel-cored aluminium conductors for overhead transmission lines. For some... 

In view of the difficulties in controlling the electroplating of speculum and bronze coatings, alternative preparation routes through the heat treatment of duplex tin and copper electroplated finishes have been proposed. ... 

Heat treatment after plating Heat treatment may be necessary after plating to improve the adhesion of coatings on aluminium and its alloys when certain processes, e.g. the Vogt process, are used, or to minimise hydrogen embrittlement of steel parts. Care is needed since heating may distort the part and impair the mechanical properties of the substrate. 

---