Frequently Asked Questions
A range of steel types with higher strength than standard low carbon, formable steels (mild steel) is available in sheet form. These high strength steels achieve their strengthening by small alloy additions and by thermo-mechanical treatment. Such materials can be used to save weight by reducing the sheet thickness without loss of component strength.
High strength steels with a tensile strength up to about 600N/mm2 are normally weldable and no modification is required in the weld time to produce welds of the required size. However, slightly higher electrode forces are usually required (typically a 20 to 50% increase) to maintain a good welding current range and process tolerance.
Higher strength steels are currently being considered for application in the automotive industry. These include: transformation induced plasticity (TRIP) steels; dual phase steels (DP); complex phase (CP) steels; and martensitic steels - with a tensile strength of up to 1400N/mm2 .
For these steels, with a tensile strength above 600N/mm2 , the electrode force required may be typically at least double that used for low carbon steels. Although welds of acceptable size can be achieved, some steels can suffer high weld hardness because of the fast quench rate in spot welding. This can cause weld embrittlement, and interface fracture can occur on testing instead of the normal plug or button failure, particularly with thicker materials. In such cases, the welding schedule needs to be changed to reduce the weld hardness. Suggested welding schedule modifications include:
- Long weld times and pulsed conditions to increase the heat in the weld area and thus slow the cooling rate
- Controlled cooling applied to the standard welding cycle by adding a low level current pulse immediately after the weld time
- In-process tempering added to the standard welding cycle by including a cool time to quench the nugget, followed by application of the temper current (for a fixed temper time)
In-process tempering may be difficult to control reliably and adds one or two seconds to the sequence time. However, this is likely to be needed for special steels such as TRIP types which have a carbon content of around 0.3%.
For steels above 1000N/mm2 , the benefits of using high strength steels could be restricted by limitations in spot weld strength caused by HAZ (heat affected zone) softening.
Weld hardness will be reduced when welding a higher carbon grade to a low carbon grade because of dilution in the weld nugget. In the same way, the addition of a thin strip of low carbon steel as a filler has been suggested.