Factors That Make Welding Stainless Steel Difficult

Factors That Make Welding Stainless Steel Difficult

 Thermal Conductivity

The thermal conductivity of stainless steels differs from that of carbon steels. For example, chromium steels conduct heat at about half the rate of carbon steels. Austenitic steels are even worse, conducting at about a third the rate of carbon steels. This means the temperature in the welded area stays elevated for longer, which in turn causes certain problems, particularly excessive shrinkage in austenitic steels.

This problem needs to be eliminated as much as possible. To do this, the amount of heat input should be kept low (by using a small-diameter electrode), and the necessary precautions should be taken during cooling (such as wrapping the weld in insulation, and similar measures).

Thermal Expansion

Chromium-alloyed stainless steels generally have the same coefficient of thermal expansion as carbon steels. Austenitic steels, however, expand about 50% more than other carbon steels. This causes large amounts of shrinkage as the weld seam cools in austenitic steels. This shrinkage increases stress in the weld zone, raising the risk of cracking. Hot cracking is especially likely to occur in double-sided internal fillet welds.

To prevent this, the same precautions taken to address problems arising from thermal conductivity can be applied here as well.

Electrical Resistance

Stainless steels have 4 to 7 times higher electrical resistance than unalloyed carbon steels. Because of this, stainless steel electrodes heat up (turn red) more easily than ordinary electrodes.

For this reason, stainless steel electrodes should be made shorter than ordinary iron electrodes and should be loaded with about 25% less current intensity.

Grain Growth

Grain growth occurs above 1150°C when welding chromium ferritic steels. While welding, part of the heat-affected zone and the weld itself reaches 1150°C, and this is where the risk of grain growth begins. In these steels, grain growth occurs rapidly, producing very large grains. These steels normally have a fine-grained, ductile structure. Once they become coarse-grained, their transition temperature rises well above room temperature, and consequently their notch-impact toughness drops significantly at the operating temperature. Once the material becomes coarse-grained, it’s impossible to restore it to a fine-grained structure through heat treatment, because no phase transformation occurs during heating and cooling.

When welding this type of steel, the process should be controlled so that the material (base metal) stays above 1150°C for as short a time as possible, thereby preventing excessive grain growth. This can be achieved with a welding process that delivers a certain amount of heat to the part and then ensures rapid subsequent cooling.

Chromium Oxide Formation

Due to the strong affinity between oxygen and chromium, a thick oxide layer forms in the weld pool during welding. Oxidation occurs very quickly, and the melting point of the resulting oxide is very high. The chromium oxide that forms so readily lowers the quality of the weld. Because of this, a special welding procedure and technique must be used to keep the weld pool and the arc isolated from contact with air.

Carbide Precipitation

Chromium has a stronger affinity for carbon than for other elements. Because of this, in high-carbon alloy steels, if carbon comes into contact with molten metal or if a shared carburizing environment is present, there’s a strong tendency toward carburization during welding.

Some austenitic steels, particularly types like 18/8, form chromium carbide when heated to, held at, or passed through temperatures between 450°C and 850°C. Under these conditions, the chromium that gives stainless steel its resistance to corrosion becomes neutralized.

When the temperature rises above 450°C, chromium’s diffusion capability increases enough to diffuse carbon outward from the grain boundaries, and due to carbon’s strong affinity for chromium, it combines with chromium to form chromium carbide (Cr4C). As a result of this carbide precipitation, a generally continuous network of chromium typically forms along the grain boundaries. Since chromium carbide is about 90% chromium by weight, even a very small amount of carbon at the grain boundaries excessively depletes the amount of chromium around the austenite grain.

As a result, chromium-nickel austenitic steels are no longer corrosion-resistant once heated between 450-850°C. When the material comes into contact with a corrosive atmosphere, it corrodes along the grain boundaries, where the chromium content is low. This type of intergranular attack can destroy the entire material in a very short time.

To prevent carbide precipitation;

• Having a lower amount of carbon available for carbide precipitation results in less carbide formation. If the carbon content is below 0.06%, the amount of carbide at the grain boundaries will sufficiently reduce the risk of intergranular corrosion, making the steel much more suitable for welding.
• Carbide precipitation can also be prevented by adding another element with a stronger affinity for carbon than chromium has. These elements are generally titanium, niobium, and tantalum.
• If carbide precipitation does occur at the grain boundaries during welding, it can still be reversed through heat treatment. The required heat treatment consists of heating the entire part to 1100°C and then quenching it in water. This dissolves the chromium carbide that has formed back into the austenite, and quenching in water prevents this carbide from forming again. However, applying such a heat treatment after welding is not practical.

Sigma Phase

The sigma phase is a very hard, antimagnetic, and brittle intermetallic compound. The sigma phase increases the steel’s macro-hardness, which leads to cracking. It also lowers notch-impact strength due to the embrittlement it causes in the steel. For these reasons, the sigma phase is undesirable in steel.

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