To effectively weld stainless steel with tungsten inert gas welding process, it is important to consider carbide precipitation.
TIG Weld Stainless Steel: Carbide Precipitation
What is Carbide Precipitation? Carbide precipitation is said to occur when the chromium and carbon in 300 series stainless steels are drawn out of the microstructure and react with atmospheric gases. It occurs between 800°F – 1650°F (426°C – 899 °). So, one must aim to keep the metal below this temperature range. Alternatively, the aim should be to keep the metal under an inert gas envelope when it is in the aforementioned temperature range.
Most annealed austenitic stainless steels such as 304 have a characteristic to undergo this microstructural change. When the metal is exposed to the temperature range of 800°F – 1650°F (426°C – 899 °), the carbon and chromium diffuse out of the crystal structure and form chromium carbides. This chromium carbide precipitates out of the solid solution and settles at the grain boundaries.
The speed with which the chromium carbide precipitates depends on several factors. The temperature of the metal is one factor. The rate of carbide formation peaks at about 1200°F (648°C). At the outer limits of the range 800°F – 1650°F (426°C – 899 °), the rate of formation of carbide falls to almost zero.
Another factor is the time of exposure of the metal to the high temperature. Understandably, the higher the exposure time, the more is the amount of carbide precipitation.
The third most important factor is the amount of carbon present in the microstructure. A higher percentage of carbon means that a higher amount of carbide formation will occur. This is why, low carbon content is a desired feature in austenitic stainless steels.
Especially, when a part is envisaged to serve in a a high corrosive medium, it is preferable to have a low amount of carbon in the filler metal to be used for making the weld. For example, ER308 is an ordinary stainless steel filler metal grade, containing carbon up to 0.08%. However, ER308L is another grade in which the composition is the same as the ER308, but the carbon content is limited to 0.03%. The ‘L’ in the filler classification designation indicates that it is a low-carbon filler.
How is the formation of chromium carbide harmful? Well, it reduces the amount of chromium available to fight off the corrosion.
Heat And Travel Speed
Generally speaking, there are three factors in carbide precipitation: heat input in the welding, travel speed, and effectiveness of shielding gas.
A high input in the welding process causes the weld metal to cool slowly, thus increasing the chances of weld metal’s exposure to a carbide-inducing temperature range of 800°F – 1650°F (426°C – 899 °). Hence high heat input is not desirable in stainless steel welding.
Travel speed is a factor that indirectly affects the heat input. A very low travel speed means high heat input per unit length of the weld. High heat input, as we saw above, is an invitation to carbide formation. So, very low travel speeds should be avoided in stainless steel welding.
The third factor is the effectiveness of shielding gas. If the coverage of the shielding gas is inadequate, it causes more exposure of the hot weld metal to atmospheric gases. This accelerates the carbide formation.
Some Methods Of Preventing Carbide Precipitation When TIG Welding Stainless Steel
- Stainless steel requires lower amperage for the same material thickness as compared to mild steel.
- Maintaining a high enough travel speed keeps the heat input under control. Maintaining a low inter-pass temperature is also important.
- Choose the correct tungsten and filler rod diameter.
This was a little something about carbide precipitation in TIG welding of stainless steel. Please share your thoughts in the comments section below.