Before Welding Wear Resistant Steel, Be Certain to Preheat
Quality wear plate contains several alloying elements in the steel, such as Nickel, Chromium, Molybdenum, Boron and a variety of others.
Notice the dark area to the left of the flame on the bucket side plate. The torch is driving the moisture out of the steel. If preheat is not used, this moisture evaporates during welding, and causes Hydrogen to go to the bottom of the weld deposit. This is the cause of under bead cracking.
The more alloys and greater the alloy content, the more important to take care when welding wear resistant steel.
One of the last things you do before you are ready to weld is preheating the steel.
Preheating involves heating the base metal, in its entirety or in the area surrounding the joint to a specific temperature before welding.
Preheating has many benefits, including reducing the risk of cracking. However over-heating wear plate often destroys the hardness of the steel.
Why Do You Need To Preheat?
The need for preheat increases with the following factors:
- The larger the mass being welded.
- The lower the temperature of the steel being welded.
- The lower the ambient temperature.
- The higher the Carbon content.
- The greater the alloy content.
- The greater the welding speed.
- The more complicated the joint shape.
- The smaller the weld rod/wire diameter.
When Should Preheat Be Used?
The following factors should be considered in determining whether or not to preheat: welding code requirements, section thickness, base metal chemistry, restraint, ambient temperature, filler metal hydrogen content or previous cracking problems.
When there are no codes governing the welding, one must determine whether preheat is required, and if so, what preheat temperature will be appropriate.
Confusion is caused because preheat is usually not required on low carbon steels less than 1” (25 mm) thick, depending on the ambient temperature.
With the addition of alloying chemistry, the demand for preheat increases with:
- Diffusible hydrogen levels in the weld metal
- Section thickness
What Preheat Temperature is Required?
Click Here to download a PDF of the JADCO preheat guide for wear resistant steels.
In production welding, codes generally specify minimum values for the preheat temperature. This may or may not be adequate to prohibit cracking in every application.
Preheat is beneficial when the risk of cracking is increased due to composition, restraint, hydrogen level or lower welding heat input.
This procedure is extremely important for high strength, low-alloy steels that have high hardenability.
Quality wear plates are included in the HSLA or high strength, low-alloy steel family. Most wear resistant steels are Quenched and Tempered to achieve the desired hardness. This is where JADCO QT Plus ® gets its name.
It is essential minimum and maximum preheat temperatures be established and closely monitored when welding Quenched and Tempered steels.
NOTE: Overheating Q&T steels may destroy the hardness of the wear resistant steel. Don’t destroy your investment. If the steel shows color, it is already overheated.
How is Preheat Applied?
The material thickness, size of the weldment and available heating equipment should be considered when choosing a method for applying preheat.
Large structural components may require multiple heating torches, electrical strip heaters, or induction or radiant heaters.
Measure the preheat temperatures not less than 3 in. (75 mm) in all directions from the weld joint.
This means you don’t measure only in the weld joint location. To guarantee the material surrounding the joint is heated, check the heat the side opposite of the area to be welded. Then measure the surface temperature adjacent to the joint.
How Long Must You Maintain Preheat Temps?
Heating may be continued during the welding process, but frequently the heat generated from welding is enough to maintain the desired temperature.
Inexperienced welders tend to rush the preheat process. If the temperature is to be 250° F, and you see the temperature is 150° F, you are not ready to weld.
The term ‘Interpass Temperature’, is defined as the base metal temperature between the first and last welding passes. Interpass temperature cannot fall below the preheat temperature.
The temperature of the steel should be checked to verify that the minimum preheat temperature has been achieved immediately prior to initiating the weld arc for each pass.