Masking Techniques (Part 2)
We continue our discussion from last month on the where, why and how masking methods are used to protect part surfaces from oxidation or limit the areas on a component part where a case-hardening process is to be performed. Let’s learn more.
For those of us who have used (and abused) stop-off paints, valuable lessons have emerged, the first of which is that each manufacturer’s product type seems to work somewhat differently. Each paint is manufactured according to proprietary formulas and, as such, their performance and protection in a given application will vary. Therefore, it is mandatory to precisely follow the advice given by the manufacturer in their technical literature and to discuss your application needs with them.
Another lesson is that no product is perfect. Just because a product works in one application, say for medium case-carburizing protection, it does not ensure that it will work for deep-case carburizing. Often, a variety of stop-off paint products are necessary. Manufacturers are also constantly striving to improve their products (and introducing new ones), making it somewhat confusing to know which product is best for a given application. Having good support from the manufacturer is critical.
There is a general “rule of thumb” that no more than 30% of the total area of the parts within a furnace load be coated with stop-off paint. Exceeding this amount can create an imbalance in the furnace atmosphere. As the parts are heated, water vapor or gases generated by thermally cracking the paint binders are released. Over time, the furnace atmosphere will recover, but the cycle disruption can influence the case-hardening process.
Product shelf life is limited (1-3 years) and dictated by the manufacturer. It is a good idea to mark the container using a simple use-by date. The paints should also be stored indoors at room temperature, and older product should be used first. Water-based paints must not be allowed to freeze.
Ideally, parts and paint should both be at room temperature when the paint is applied to clean, dry surfaces. (Caution: A clean, dry surface is essential to proper adhesion.) After thoroughly stirring the paint, the coating should be applied as uniformly as possible in a way that the surfaces to be protected are covered completely and the part surface is completely hidden from view. Some stop-off paints are diluted before use. This should not be done arbitrarily but by following manufacturer’s instructions, and if this is done, care must be taken to avoid a situation where the paint runs onto unintended surfaces during application. Multiple (2-3) thin coats are often applied, but excessive coating thickness must be avoided. A common misconception is that the thicker the coating, the better the protection. This is not the case.
Required drying time is dependent on factors such as paint composition, viscosity, coating thickness, ambient temperature and part temperature. Drying time is significantly influenced by the relative humidity in the room. Typical drying times vary from a few hours to several shifts. Solvent-based paints will dry relatively fast due to the accelerated evaporation of the solvent. Drying time for aqueous-based paint can be shortened by placing them in an air oven or furnace at a maximum temperature of 180˚C (350˚F). Exceeding this temperature could cause the paint to flow onto uncoated areas as well as compromise the effectiveness of the paint. One of the consequences of inadequate drying time is blistering or peeling (Fig. 1), which may burst and throw stop-off onto uncoated areas and cause spotty case hardening.
Recognize that many stop-off paints are hygroscopic, meaning they will absorb moisture as they sit around. It is recommended that painted parts be heat treated within 24 hours of the last coat. In high-humidity environments, it is recommended to store painted parts in an oven running no higher than 80°C (175°F) until they are used.
The painted areas of the part must be racked to ensure these areas do not come into contact with uncoated areas of other parts in the furnace load. It is also recommended that you do not use different types of stop-off paints in the same furnace load. This situation could compromise the effectiveness of one or both of the products, especially if a solvent-based boron paint and a water- or silicate-based paint are in the same load. Water- and silicate-based paints give off water vapor during the heat-up cycle, which can attack the solvent-based paint and cause it to run onto uncoated areas.
After heat treatment, it is recommended that the residue of the paint be removed promptly. The residue of some stop-off paints can react with humidity and cause a corrosive attack of the part surface. Many users are overly concerned with ease of removal and select stop-off paints based on this criterion alone. This decision must be carefully weighed against the protection afforded by the product.
In general, stop-off paints are effective for only one heat-treat operation (e.g., hardening followed by quenching). There is one exception to this rule, namely certain silicate-based paints that will prevent carbon penetration not only after carburizing and slow cool cycle but through reheat and subsequent oil or gas quenching. The parts can be cooled further in air to room temperature. These same parts can be reheated in atmosphere and oil quenched for the hardening cycle.
Most of the problems encountered when using stop-off paints are well known and preventable. These include:
Adherence: Improper paint adherence is almost always traced back to surface cleanliness, insufficient drying time or poor application techniques.
Blistering/Peeling: Insufficient drying time is another leading cause of stop-off paint failures. All stop-off paints utilize a thinning agent, which must be fully evaporated prior to introduction into the furnace. Insufficient drying will cause the thinning agent to outgas, lifting areas of the paint off the part surface (blister) during heat-up.
Incomplete Coverage and Edge Effect: This most often occurs because of miscommunication as to which surfaces need protection or which areas need coverage. Good communication often negates these concerns. Unwanted case at the edges of the painted surface is also a concern overcome by extending the edges of the painted surfaces (if allowable).
Product Misapplication: One of the most catastrophic problems with stop-off paints is the choice of the wrong product for the job.
Furnace Damage: Damage to the furnace interior has also been traced back to the use of certain stop-off paints. Common problems include deposits on the tips of oxygen probes (skewing carbon-potential readings) and deposits on the refractory surfaces, causing “glazing” (Fig. 2). For example, boron oxide is known to combine with the water vapor present in endothermic-gas atmospheres to form boric acid. The boric acid reacts with the silicon in the refractory, causing a eutectic. This phenomenon normally happens over a long time period, and deposits may occur in areas of temperature transition (inner doors, etc.).
Both solvent- and water-based boron stop-off paints break down during the heat-treat cycle. For a water-based stop-off, chemically combined water will evolve as low as 190˚C (375˚F). Resins and binders in solvent systems evolve around 360˚C (680˚F). It is a time-temperature relationship.
Excess Stock Allowance
This is the most-costly and least-preferred masking method and involves considerable post heat-treat time and expense (post machining). For certain areas of parts where the geometry is highly distortion-prone, however, adding extra stock provides selective carburization while minimizing distortion.
Effective masking techniques are those that work. While experience is the greatest teacher, understanding what level of protection is needed and how best to achieve it before running component parts is critical. Million-dollar mistakes have occurred because of poor plating adherence, failure to properly clean part surfaces and using the wrong product for a given application. Get the facts and learn from others’ mistakes so as to avoid your own. 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.