December 11, 2003
Welding can severely influence strengthened or hardened metals, depending on the hardening technique used. Because of this, post-weld heat treatment is often very helpful in maintaining weld joint strength because it softens or tempers any martensite or bainite that formed in the HAZ.
As I mentioned in the September/ October issue, welding can severely influence strengthened or hardened metals, depending on the hardening technique used.
Work- or strain-hardened metals exposed to the intense localized heat of welding tend to recrystallize and soften in the heat-affected zone (HAZ). Assuming the correct filler metal is used, the only area affected is the HAZ. The admixture and filler metal don't suffer recrystallization and remain as strong as the base metal. This explains why, when you're dealing with work- or strain-hardened steel, failures usually happen in the HAZ right next to the weld joint, not directly in the joint.
This is especially true for cold-rolled steel, wrought iron, and drawn or rolled aluminum. When working with these materials, joint design is critical, and you must take into account the amount of stress the finished piece will experience in service.
Precipitation-hardened metals go through a more complex change than work-hardened metals do, but the end result is similar, with the HAZ going through an annealing cycle and becoming softer. That's because the precipitate that gives the metal its strength grows and agglomerates with heat—it over-ages. This reduces the effects of precipitation hardening. The higher the heat, the faster the metal reaches the over-aged, or weakened, state. Post-weld heat treatment can correct this, as long as you carefully select the filler metal to match the base metal's aging characteristics.
Metals that have been solid-solution-hardened have the least amount of change when welded. There's a little grain growth at the fusion line, but usually not enough to have any effect on the metal's properties.
Transformation-hardened metals react much like solid-solution-hardened metals, assuming they have enough hardenability to form martensite during heat treatment or have formed martensite in previous heat treatments. A temperature profile of a transformation-hardened metal identifies four basic regions in the HAZ, with heat input determining both the width of the HAZ and the width of each region.
The higher the heat input, the wider the HAZ and the slower the cooling rate. Slower cooling rates are less likely to form martensitic regions. Consequently, you can reduce post-weld brittleness by preheating to slow the cooling rate, although you may also have to post-heat the weld to slow cooling further. This also means that the harder the HAZ, the more martensite, and the more martensite, the greater the chance for cracking.
Because of all this, post-weld heat treatment is often very helpful in maintaining weld joint strength because it softens or tempers any martensite or bainite that has formed in the HAZ. It also relieves stresses that can lead to cracking.
In fact, proper heat treatment can change grain size; modify the ductility, hardness, toughness, or tensile strength; improve magnetic or electrical qualities and machinability; relieve stress; recrystallize cold-worked metals; and even modify the chemical composition and properties of the metal's surface (case hardening).
The key is to perform the heat treatment properly: There's more to it than taking a torch to the steel and then letting it cool for a while. The critical heat treating factors are what you might expect: temperature, time, and cooling rate. Of course, the chemical composition of the surrounding materials also influences effectiveness.
When it comes to controlled heating of the metal, there are a number of ways to do it, including oxyfuel or fuel-air torches and temperature-indicating crayons, furnace heating, induction heating, electrical resistance heating, natural gas, or electrically heated salt or molten metal bath.
There are also a number of methods for controlled cooling, including gradual furnace cooling, cooling in still air, cooling in agitated air, fan cooling, water cooling, and allowing the metal to cool buried in sand.
But in terms of heating and cooling, control is critical. That is, being able to control how slowly (or quickly) the part is heated, as well as the temperature it's heated to, how long it's held at that temperature, and how long it takes to cool back down to room temperature. And the specifications for all of those variables depend on not only what the metal is, but also what you want the heat treatment to accomplish.
For example, you may want to soften a metal to make it easier to machine or cold work or to relieve internal stresses from welding or forming. This is done by annealing, basically a four-step process that involves:
The heat-soak equalizes the temperature throughout the metal and turns it completely austenitic. As it cools very slowly, the austenite transforms to ferrite and pearlite, and the metal reaches its softest state, with small grain size, good ductility, and excellent machinability.
Normalizing is another heat-treating technique that's often used to prepare a metal for future heat treatments. Normalizing can increase uniformity of a metal's internal structure, improve ductility, and reduce internal stresses. And while it does make the metal softer, it doesn't make it as soft as full annealing. Normalizing involves heating the metal to slightly above its A3 temperature, holding it there for austenite to form, and then slowly cooling it in still air.
Thermal stress relieving is just what it says: a heat treatment to relieve internal stress. It involves heating the metal to a temperature below the lower transformation temperature (A1), holding it there long enough to relieve the locked-up stresses, and then cooling it slowly. This is sometimes called process annealing.
For stress-relieving steel, the most common temperature range is between 1,100 and 1,150 degrees F. This is high enough to decrease the yield residual stresses by 80 percent, yet low enough to prevent any metallurgical changes in most steels. You can get up to 90 percent stress relief by heating the metal to just under the critical temperature, but some steels can become brittle after thermal stress relief at these temperatures.
That covers the basics of how welding affects heat-treated metals and some of the ways we can counter those effects with heat-treatment techniques. Next time, we'll get back to some theory and begin a close look at the specifics of welding metallurgy.
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