Contributed by Menachem Kimchi, Ohio State University
Advance High-Strength Steels (AHSS) have been resistance welded in automotive production lines in the last few years. However, the high strength and hardness can be expected to affect spot weld failure modes during the typical peel testing and chisel testing performed for weld quality evaluation.
A well-established industry standard associated with peel testing of conventional steels is that an acceptable test is one in which the peel test “pulls a nugget” or a “full button,” as shown in Figure 1.
However, with AHSS, full button pulls are less likely due to the high Carbon Equivalent that are likely to produce hard weld nuggets. This fact is compounded by the higher yield strengths of the material that will tend to produce greater stresses concentrating at the edge of the nugget during a peel or chisel tests. Therefore, the conventional modes of testing such as peel and chisel testing may be more likely to produce interfacial or partial interfacial failure modes such as those shown in Figure 2.
With AHSS, these types of failure modes may occur even though the weld strengths may be acceptable for the intended application. Even full interfacial failures may exhibit high strength, although it may sometimes be challenging to differentiate between an interfacial failure and a “stuck” weld condition (which refers to an unfused bond of unacceptable strength).
To improve nugget failure modes with AHSS, the hard martensite must be softened. A simple and effective approach to accomplishing this is to add a temper cycle at the end of the weld (Figure 3), something that can easily be added at the end of the spot welding cycle. A sufficient amount of quench time must be included prior to tempering to allow the complete transformation to martensite, and the amount of tempering time and current will dictate how much softening occurs. Of course, the quench time adds cycle time to each weld, so production requirements mandate that this time be kept as short as possible. Depending on how hardenable the steel being welded is, other approaches that slow cooling rates may be helpful. These approaches include current pulsing, current sloping, longer weld times, and short hold times.
Sir if the spot weld load is 26 KN. In 308 ss 2+2.5 mm thickness. And there is a interfacial fracture in the joints. It is okay or not because with increase in carbon the brittleness property will increase and cause the fracture. It is acceptable fracture or not???
You are talking about a Stainless Steel, are we correct? This blog is about Advanced High-Strength Steels. We would be happy to help you if we can, but we would need much more information about the process in order to address your concerns.
Post PHS parts can be mechanically joined using a localized heat treatment process to soften only the area required flow to create an acceptable mechanical interlock using typical clinching methods and Self Piercing Rivets without the interfacial failure modes created during resistance spot welding (RSW). These joints have exceptional cross tension strength that typically exceed the strength of RSW. The process was developed by a tooling supplier in Michigan working closely with ArcelorMittal to help optimize the heating and joining requirements utilizing PHS up to 2000 MPa.
Mark, a great example of the steel industry working with suppliers and OEMs to solve issues. WorldAutoSteel is working on RSW issues as well, and we are hoping to begin reporting our findings in 2019.
The AHSS resistance spot welding should be request individual parameters settings. The DP, CP,TRIP and PHS are totally different. The nugget diameter does not warranty good performance. We need to started thinking in Carbon equivalent and Hydrogen embrittlement.