Assemblies designed to take full advantage of KVA STAINLESS™ processing technology can benefit from utilizing high-temperature furnace brazing in their design. Brazing, a joining process whereby a non-ferrous filler metal or alloy is melted and distributed between a close-fitting joint interface by capillary action, can be accomplished reliably, repeatably with low operator skill, low cost and no loss of base metal strength in a simultaneous furnace hardening/brazing cycle.
In the shift to higher strength alloys in order to improve product strength and/or reduce product weight, many industries are experiencing fabrication and manufacturing problems, particularly related to welding and joining. Higher strength steels typically suffer from limited weld process windows with restricted heat input and weld speed rates. Even with optimum weld parameters, microstructural changes occur in the weld and heat affected zone (HAZ) resulting in a loss of part strength. These changes in material properties are exacerbated by the fact that the weld toe serves as a stress concentrator, magnifying loads on the compromised HAZ microstructure. High strength alloy part failures in the weld-HAZ-base metal interface are not uncommon, resulting in reduced productivity and product performance.
Realizing the potential of utilizing martensitic stainless steels (MSS) in structural components and assemblies, KVA STAINLESS™ has developed novel joining methods to use these existing materials in exciting new applications. These technologies are the result of decades of metallurgical and thermal processing R&D and know-how by KVA STAINLESS™' founder, Mr. Ed McCrink, and development staff. KVA STAINLESS™' proprietary, simple to implement methods have overcome conventional difficulties enabling low-cost martensitic stainless steel (MSS) products.
During the hardening heat-treat cycle for MSS, components can be joining using properly selected filler alloys and joint designs. The resulting interfacial bond, properly designed and implemented, can be made stronger than the hardened MSS base metal. Assemblies can be hardened and joined to form uniform, high-strength structures, with no loss of strength in areas adjacent to the braze joint - as is seen with conventional welding methods.
Key advantages of KVA STAINLESS™ brazing technology:
KVA STAINLESS™ simple to implement methods eliminate conventional joining difficulties, such as welding operator skill requirements and part-to-part variation. The braze material can be easily and precisely applied to the part in pre-form or paste form prior to heat treatment. Dissimilar metals can be brazed together with proper process and atmosphere controls. Additionally, brazed MSS exits the furnace with a bright, clean appearance, free from unsightly oxidation and scaling that often accompanies welded stainless joints. Fatigue performance can be improved, due to the smooth transition from part-to-part made by the flow of the braze filler alloy Ð unlike conventional weld beads. Most importantly, secondary joining processes can be eliminated, saving assembly time and cost, while improving product quality and performance.
Any mechanical straining after welding (i.e. continuous tube mill forming /straightening) will cause the martensitic HAZ to crack. Conventional processing methods for martensitic weldments, such as pre-heating and lengthy post weld heat treatments (PWHT), do not lend themselves to cost-efficient, high-quality, high volume production.
Permanently joining parts (of the same, similar, or dissimilar materials) by brazing them in a furnace, under either controlled-atmosphere or vacuum, is a very cost-effective method for manufacturing simple or complex assemblies in production quantities, limited only by the physical and chemical properties of the materials themselves and the size of the assembly relative to the furnace.
Brazing does not deform or weaken the assembly, and the use of chemical fluxes and post-joining cleaning operations is eliminated or minimized. A high degree of flexibility in atmosphere selection and blending allows precise control of the factors which most influence braze quality, primarily removal of surface oxides, dewpoint control, carbon control, and wettability. Results are reproducible and compatible to accepted quality control techniques, and special operator skills are not required. Considerable attention must be paid to the selection of base metals, filler metals, joint design, fixturing, and atmosphere composition - KVA STAINLESS™ has extensive knowledge base for product success.
The characteristics of furnace brazing filler metals to consider include:
The most common atmospheres used in controlled-atmosphere furnace brazing operations are classified as exothermic, endothermic, dissociated ammonia, and industrial gas-based (generated or delivered). What all of these atmosphere types have in common is that they are used for moderate- to high volume production applications, mostly in a continuous or semi-continuous (retort or bell) furnace. They can also be used in vacuum furnaces, as a source for inert, purge, or backfill gas.
All of the brazing atmosphere types reduce oxide formation after precleaning and control the formation of oxides during brazing. They help to control wettability and braze flow, and assist in optimal microstructure formation.
Perhaps most importantly, controlled-atmosphere brazing eliminates the need for fluxing in most applications, which means lower labor costs since parts can be finish-machined or used immediately without post-braze cleaning. Also, the absence of flux residue is a benefit for parts with complex geometries where flux can become entrapped, or threaded holes where complete removal of flux is difficult or impossible.
The unique air-hardening property of MSS allows for simultaneous hardening and joining to be performed, with minimum distortion due to the slow cooling rates involved - versus conventional water or oil quench methods which induce significant thermal shock, distortion and residual stresses on parts. The result is that heat-treated MSS can now be used in a wide variety of structural applications, without significant cost increases.
Ideal applications for KVA STAINLESS™ brazing technology include: