Home

 

Summer 2024 Contents:                                                                                                Issue No. 63

________________________________________________

Tempering of Resistance Welds

In this nugget, we review tempering of resistance welds; a post-weld heat treatment step to reduce weld hardness.

Click here to download an article on Tempering of Resistance Welds

(Below is text of the article without figures; if you would like to download pdf copy with figures and tables, please click on the link above)

________________________________________________________

In the previous edition of this newsletter (Spring 2024), we reviewed hardness and hardenability of steels.  Hardness in the weld zone is not only a function of composition but also of cooling rate, and resistance welding is a process with one of the fastest cooling rates due to cooling by the welding electrodes.  While some level of hardness is preferred as it increases the strength of the weld, an excessive increase in hardness can be detrimental to the fatigue life of the weld. In such situations, a pre- and/or a post-weld heat treatment has to be provided to reduce the hardness to manageable levels. In this newsletter we will review the implementation of a post-weld tempering heat treatment for a resistance welding application in order to control weld hardness.

 

Resistance welding is a process that depends on heat produced by passage of current through the parts to produce desired heating (See Resistance Welding playlist on YouTube channel The Weld Nugget).   It is a contact process and typically uses water-cooled copper electrodes to apply welding force and current.   In addition, the copper electrodes also act as a heat sink and remove heat from the weld that results in a very rapid cooling rate at the end of the weld.  The net result is that resistance welds can have high hardness in the weld and may require a post-weld heat treatment called tempering.  The cooling rate required to form martensite for a particular grade of steel may be readily available in the form of a TTT (Time Temperature Transformation) diagram or Continuous Cooling Transformation (CCT) diagram (TTT and CCT Diagrams for Carbon Steels)  Tempering is a process that raises the temperature of the weld zone to a level where martensite formed during rapid cooling is transformed into softer ferrite and pearlite.  

 

Given the complex size and shape of welds, it is often not easy to determine the exact cooling rate in the weld, and hence the TTT and CCT diagrams are not always directly helpful.  Instead, it is better to make welds and send them for metallurgical analysis where the lab personnel will section the welds and run a hardness scan to evaluate hardness changes across the weld.  For example, weld section shown in Figure 1 for a 0.2% carbon steel was evaluated for hardness and indicated that the hardness in the weld zone was slightly over 50 HRC, which matches the maximum hardness (within margin of measurement error) that can be expected in such a steel, according to the hardness chart (Spring 2024).   Result indicates that the weld was cooled at a rapid enough rate to form the maximum amount of martensite possible.

                    

Figure 1. Weld section of a resistance projection weld, along with hardness trace after welding; also includes hardness trace of another sample that was welded and was post-weld heat treated (Tempered) showing a drop in hardness.

 

 

Hardness levels greater than 40 HRC are typically considered undesirable in a weld, and hence a weld with 50 HRC would be an ideal candidate for a post-weld tempering process.  One of the benefits of resistance welding is that the process is well suited to provide post-weld heat treatment as a second pulse while the part is still in the welding station.  The second pulse will raise the temperature to a higher level where the martensite is transformed to ferrite and pearlite, but not so high that the martensite is transformed back to austenite.  Here again, direct measurement of post-weld treatment temperature is not practical, and hence we depend on weld trials and hardness measurements to gauge effectiveness of the tempering process.  Hardness scan data after tempering (Figure 1) shows that the maximum hardness in the weld zone has dropped by 10 HRC to close to 40 HRC as desired.  Keep in mind that this projection weld produced a solid-state bond and does not have a fusion zone (Types of Bonds in Resistance Welding).   Hardness in the vicinity of transformed zone also has reduced by about 5 HRC, and that is likely due to annealing of work hardened material from prior manufacturing steps.
 

 

Figure 2.  Schematic showing the tempering process on a TTT diagram: 1. Cooling from weld pulse, 2: Hold time between pulses, 3: Heat to Tempering Temperature, 4: Cool down to room temperature.

 

 

A schematic TTT diagram is shown in Figure 2, where the individual steps and transformations during welding are indicated.  Fact that the weld in our experiment did achieve maximum hardness after welding indicates cooling at end of the weld (Step 1) was fast enough to miss the nose of the TTT curve.  Also, the tempering cycle (Step 3) would have reached sufficient temperature and time as it did produce the desired reduction in hardness.

                                                                                                    

On one hand, resistance welding is likely to produce the highest hardness for a given steel composition due to rapid cooling, but on the other hand it also offers the convenience of providing post-weld heat treatment as part of the welding sequence itself; two of the many unique attributes of the oldest electricity-based welding process and a testimony to its continued popularity.