Resistance Welding of Hot Rolled Sheet– From “Bumper Component Welding State-of-the-Art Survey” by D.W. Dickinson
AISI Bumper Project Group,Dec31, 2000
This Figure is a schematic representation of the spot weldability comparison of hot rolled steels used in bumper manufacture. The figure consists of 10 axes, each one representing a grade of steel. The axes are labeled weld time and weld current in order to symbolize a weldability lobe type diagram. However no values are on the axes as there are many welding parameters that cause an actual weld lobe to vary size, shape and position thus invalidating an actual data comparison. The symbols, therefore, rather than being actual lobe curve data are icons that represent symbolically the size of the weldability lobe curve and in a relative fashion, its sensitivity to the welding parameters for each of these steel grades. This representation is somewhat arbitrary, but it does offer the ability to generalize a rough comparison between various grades of steel. The icon itself looks like a three dimensional diagram. The front face (the largest face) represents a base line of weldability and the depth represents sensitivity to some weld parameter. Thus, a wide-open icon indicates that grade of steel generally is weldable over a wide range of parameters. On the other hand, a restricted icon represents a steel grade that is weldable but weldability losses are expected with changes in parameters. The greater the restriction in the icon, the more the weldability loss might be expected. Axes where there is no icon indicate that a sufficient volume of spot weldability information was not available to represent the weldability of that material; it does NOT mean that grade is unweldable.
The CQ and DQ material are the baseline of resistance spot weldability. They are weldable over a wide range of welding currents and over large weld times. They are only minimally affected by changes in electrode force. (Changes in electrode force effects contact resistance and thus higher electrode forces result in lower initial contact resistance and thus higher currents or longer weld times are required to obtain the same size weld nuggets). However, the current range over which acceptable nuggets are produced although moved to a different level is hardly effected at all. Weld hold time after current flow (where nugget solidification occurs) does not cause production of “brittle” (this is a term used in the industry indicating weld centerline or interfacial failure but the failed microstructure may not necessarily be of a brittle nature) weld nuggets which experience nugget interfacial failure upon testing as does some higher carbon steels. Tensile loaded weld experience high ductility on tearing although they do not have extremely high total strength. Seldom does interfacial failure occur as a result of centerline inclusion or porosity. The DQ steels that have received special treatment to lower total inclusion content and modify inclusions remaining are somewhat better in centerline performance because of their cleanliness.
The 35XLF, 50XLF and 55XLF steels also have excellent weldability closely comparable with that of the DQ material. Steels in this strength range appear to not suffer deleterious spot weldability effects as a result of the slightly higher strength with the possible exception of the rephosphorized paint-bake hardenable steels. The phosphorous in these steels tends to segregate to the centerline of the solidifying weld causing some centerline hot tears thus rendering these steels slightly more susceptible to “brittle” interfacial nugget tears. Thus the paint-back hardened steels find little use in bumpers.
The high strength 80 XLF materials obtain their strength from the controlled rolling grain size control and precipitation mechanisms described above for the HSLA type steels. There are a number of microalloying routes that can be applied in order to obtain the grain pinning and precipitation strengthening. Also as mentioned above, these precipitates have varying stability under the reheating from the weld thermal cycle. When overaging of the precipitates or dissolution of these precipitates occurs, grain growth strength loss and loss of precipitation strengthen may occur. Often multiple additions of two or more of these precipitate formers are added in order to minimize and heat affected zone strength loss. In any case this is considered a minimal effect and does not hinder the application of these steels in most resistance welding applications. In other respects, these steels are employed because they closely simulate the fine resistance weldability of the CQ and DQ materials.
Weldability tests run on the hot rolled dual phase steels falling in this high strength level range are generally favorable and respond very similarly to those of the other HSLA steels in resistance welding. The steels produced by the annealing routes prove most weldable, however in some cases the as rolled dual phase, with its higher chemistry, has shown some variability. The sister materials, the TRIP Steels, are relatively new grades and insufficient weldability data has been collected on these at this time.
The ultra high strength M190 HT steels get their strength from quench hardening of somewhat higher carbon material to martensite. These materials have very high strength but limited ductility. Thus the resistances weld nuggets (that are solidified and cooled very rapidly as a result of the water-cooled electrodes) form hard brittle martensite. They tend toward nugget cracking failures, and tend to be hold time sensitive. In addition because of the high strength of the base material the welds tend to be sensitive to electrode force variations as some of the force is required just to hold the weldments. In addition, the heat-affected zone thermal cycle can cause a tempering of the base metal martensitic structure and result in some HAZ strength loss. These materials are considered resistance weldable but some precautions need to be applied.