Stainless Steel Brazing and Heat Treating
Our customers have told us about many of the fabrication challenges of stainless steels. Manufacturing processes offering more precision should be used to make parts where the outcome results in a higher performance standard. Stainless parts must be strong, corrosion resistant, and have a bright finish. Making this process more challenging is the increased cost of stainless steel raw materials—leaving no margin for error, due to the exorbitant price of these materials. The underlying issues stemming from the brazing and heat treating of stainless steels are no different than these fabrication challenges.
Of course, stainless steels require superior furnace atmospheres for maximum brazed joint strength. Superior atmospheres with robust furnace heating and cooling cycles are necessary to maximize the corrosion resistance properties and to achieve proper grain structure. However, the challenge of successful stainless steel brazing extends beyond just the furnace. Weaknesses anywhere in the stainless steel brazing process usually result in scrap. Parts that fail to braze properly on the first pass are often not recoverable. The total cost of stainless steel scrap from inferior processes often approaches or exceeds the total cost of brazing (the value of parts is often ten to twenty times greater than the brazing price).
Franklin Brazing developed the PuroBrite™ process to meet the strength, corrosion resistance, appearance, and the yield and efficiency requirements of stainless steel brazing and heat treating. PuroBrite™ is a complete system that starts with you (the customer) and ends with a finished part. Weaknesses in multiple steps of the process will have a compounding effect on yields and increased costs. PuroBrite™ addresses all phases of the stainless steel brazing process and only Franklin Brazing has this specialized process.
Why is Stainless different?
Brazed joints must be properly designed – Braze alloys are more free-flowing with stainless steels, so extra care and attention is needed to keep the alloy in the joint. Our experience producing 99%+ first pass yields with many different parts allows us to match physical joint and braze method design from the beginning. Poor joint design can destroy yields before the parts hit the furnace. A thousandth of an inch could be the difference between the success and failure of joint brazing. A weakness within the brazing joint design process can result in a failed braze and/or unsightly appearance.
Parts must be clean – Stainless steels aggressively form oxides in the presence of oxygen, hence its corrosion resistant properties. Unlike mild steels, stainless steels will form oxides with the slightest foreign element at furnace temperatures. The oxidation could inhibit the formation of the metallurgical bond between the alloy and stainless base material. These oxides formed at high temperature also stain the parts. We educate both our suppliers and customers about the importance of cleanliness. Negligence toward the importance of the proper cleaning of components will result in no braze, or unsightly appearance and inconsistent yields.
Assembly methods must be designed and controlled to manage characteristics critical to brazing success – Stainless steels have strict joint and cleanliness requirements, along with final assembly dimensional requirements. Joint tolerances can be altered through improper assembly techniques, especially if thinner gage material is being used. Component parts could become contaminated in the assembly and handling process. Once assembled, the parts must maintain their assembled strength and position until the permanent braze is formed in the furnace cooling cycle. Our experience assembling millions of critical stainless steel brazed components with 99%+ first pass brazing yields is unique.
Atmospheres must be robust – The chromium in stainless steel forms a more stable oxide at a much lower oxygen level than iron. The reduction of such oxide and the prevention of parts from oxidation at an elevated temperature can be achieved in the furnace only with a pure reducing atmosphere and an extremely low concentration of vapor level. PuroBrite™ uses pure hydrogen atmospheres from cryogenic storage. This atmosphere is a key building block for protecting the corrosion resistance of stainless steels. Weak atmospheres compromise brazed joint strength, corrosion resistance properties, and the aesthetic appearance.
Heat cycle must be well controlled – Many stainless steel components have higher transformation temperatures than mild steels, increasing process risk. Stainless components must be quickly and uniformly brought to brazing or transformation temperature. Uniformity is critical to the metallurgical quality of all stainless steels. PuroBrite™ uses only continuous furnaces for optimum temperature control, uniformity and efficiency. All parts are brought to the correct temperature under precise conditions. Glitches in the heat cycle could create an unacceptable variation in finished parts and increase defect rates.
Cooling cycle must be robust and controlled – The vast majority of stainless steel grades are susceptible to inter-granular corrosion caused by chromium carbide precipitations formed on grain boundaries. This corrosion could result from the improper cooling of parts after annealing or brazing. This process leads to chromium depletion inside the grains, which significantly reduces the corrosion resistance of stainless steel. This could also lead to cracking, especially when the stainless steel is exposed to cyclic stress. PuroBrite™ includes a robust active cooling cycle that rapidly cools parts through the critical transformation range. Protective atmospheres are maintained throughout the cooling cycle to prevent oxidation. Any malfunction in the cooling cycle could destroy the corrosion resistance properties of stainless steels and compromise the overall strength of the base material. These types of defects are often latent to the naked eye.
Bright finish must be achieved – A bright finish on stainless steel parts exiting the furnace is evidence of maximum corrosion resistance. The bright finish is visual confirmation of the invisible oxidation layer formed when the clean surface of the stainless steel meets the ambient oxygen at room temperature. Foggy, grey, or other colors or tints are evidence of heavy oxidation that creates openings in the grain of the stainless steel. Only perfectly bright parts meet their corrosion resistance potential. PuroBrite™ is the only brazing or annealing process that guarantees the consistent brightness of brazed and annealed parts.