Masking Techniques (Part 1)
One should not assume that any masking method has been properly applied or will work well for a given application without testing and taking into consideration such factors as material, part geometry, cleaning, application, drying, heat-treatment process, removal method, inspection and end-use performance.
Copper, properly applied, is generally considered the best method of part protection. It is also the most costly. Copper plating can be used in both atmosphere and vacuum carburizing. AMS 2418 Rev H (2011) details the process requirements. A nickel strike (nickel flash) is often used as an under layer for improved copper adhesion, especially when processing highly alloyed steels (e.g., Pyrowear 675, M50Nil, Vasco X2). At high hardening temperatures, however, nickel has been observed to diffuse into the base metal, which has the same appearance under the microscope as a decarburized surface layer.
Platinum and palladium plating are also reported to work well for specialized materials (e.g., titanium) but are extremely expensive and should only be considered if the project cost warrants such expense.
Stop-off paints are widely used throughout the heat-treat industry either as an alternative to copper plating or for touch-up on plated surfaces that have been damaged.
Carburizing and Carbonitriding
The most commonly used stop-off paints for carburizing and carbonitriding are solvent- or water-based coatings with copper or boron as the main ingredient. Paints containing copper are not particularly suited for carbonitriding because of the possible chemical reaction with ammonia in the furnace atmosphere. The biggest advantage of boron-based paints is that the residues that remain are soluble in hot water and alkaline solutions. To a certain degree, they “rinse off” during quenching. These paints are the best choice for applications where mechanical cleaning is not a viable option.
Boron-based paints go into a semi-liquid state at temperature when heated. If the coating is too thick, the mass of the paint will cause it to run onto uncoated areas. It is for this reason that the paint thickness should be limited to 0.2-1.0 mm (0.008-0.040 inch). Only one coat is required as long as this coating is uniform. Thicker is not better. Two coats are only recommended for geometries prone to carburization from two sides, such as threads.
Solvent-based paints containing boron oxide are still in common use today and are considered by many to offer the best protection in this class of coatings.
For total case depths over 2 mm (0.080 inch), silicate-based paints are recommended over boron-based paints (Fig. 1). Silicate-based stop-off paints are normally applied in two or three layers depending on the required case depth. After heat treatment, the glass-like residue is not water or solvent soluble but must be removed by blasting. An advantage of these paints is that they will not run even if the coating thickness is excessive. Unlike water-based boron paints, they are not subject to the problem of glazing of the furnace interior.
In general, stop-off paints used for vacuum carburizing are similar (but not identical) to those used in atmosphere carburizing, especially where acetylene is used as a hydrocarbon-gas choice. Boron paints are used for applications requiring the paint to be washed off after heat treatment.If mechanical removal (shot blasting) of the stop-off paint is acceptable, silicate-based copper-oxide paint should be the first choice.
Gas Nitriding and Nitrocarburizing
Stop-off for gas nitriding and nitrocarburizing (Fig. 2) contain fine tin powder dispersed in a lacquer, consisting of a solvent and synthetic binder or water and synthetic emulsion. The stop-off effect is based on a layer of molten tin dispersed onto the part surface, which acts as a gas-
tight barrier that prevents the diffusion of nitrogen. It must be noted that preheating coated parts in air must be limited to 380˚C (715˚F) maximum. Exceeding this temperature limitation will prove detrimental to the uniformity of the tin plating. After processing, powdery residues that can be easily removed by wiping or brushing remain. It must be noted that there is a microscopic layer of tin left on the part surface. In the event this is problematic, blasting or machining can remove it.
Plasma (Ion) Nitriding
For ion nitriding, the most commonly used stop-off technology is mechanical masking (shielding). If the geometry of the part does not lend itself to this type of protection, stop-off paints are available either based on copper (electrically conductive) or ceramic ingredients (nonconductive). The residues of these paints are powdery and are most often removed by wiping or brushing.
Stop-off paints are available to prevent scale and oxidation in furnaces that are running processes such as annealing, normalizing and stress relief in air or products of combustion. These paints are used for annealing, stress relieving, normalizing and hardening. In these applications, the entire part surface is coated to prevent scale from forming when heated up to 850˚C (1560˚F). There are also lacquers and ceramic-based coatings available for applications up to 1200˚C (2200˚F). These coatings create a glass-like barrier to prevent the scale from forming. Upon cooling, the coating will begin to spall off due to differential expansion of the coating and the part surface. Mechanical removal after processing is usually still required.
Brushing, although labor intensive, is perhaps the most common application method for stop-off paints. Using a flat, clean brush with soft bristles, the paint should be applied in an even, thin layer of uniform thickness. When applying the paint to the part, resist the temptation to put excessive pressure on the brush, and let the paint flow off the brush in a uniform manner. If the paint rolls off the part surface and back to the brush, there is oil or some contaminants on the surface that must be removed. If solvent-based stop-off is used, storing brushes between coats must be done in a container with the same solvent as the paint to ensure there is no adverse chemical reaction or paint contamination.
Dipping (part immersion) is the simplest way to coat large numbers of parts. If the area to be coated is at the end of the part, semiautomatic or continuous coating can be achieved with minimal investment in equipment.
Automatic dispensing and spraying can also be used if the part geometry and area to be masked are conducive to this method. Robotics and automated handling systems can be employed for high-volume applications (such as in the automotive industry). If spraying is used, it must be carefully controlled to prevent overspray and misting into the air.
Part 2 will discuss common problems with various types of masking methods and presents valuable lessons learned. IH
1. Burgdorf, Eckhard H., Manfred Behnke, Rainer Braun and Kevin M. Duffy, “Stop-off Technologies for Heat Treatment,” ASM Handbook (in preparation), 2013.
2. Nüssle GmbH & Co. KG, Nagold, Germany (www.burgdorf-kg.de), private correspondence.
3. Duffy, Kevin, The Duffy Company, (www.duffycompany.com), private correspondence.
4. Herring, Daniel H., “Industry Practices Report, Selective Carburizing Methods,” white paper, 2004.