What Causes Carbide Precipitation in Stainless steel?

What causes Carbide precipitation in stainless steels.

First off ..what makes stainless steel corrosion resistant?

Mainly, it’s the chromium.  

One of the most commonly welded grades of stainless is 304L which contains approximately 18 % chromium. 

if you have every heard of 18-8 stainless, 304L is one of those.

The L in 304L stands for low carbon.

304L stainless steel contains a very low percentage of carbon, typically around 0.03% or less. This low carbon content helps to minimize the risk of carbide precipitation.

So what is carbide precipitation?

Carbide precipitation is a detrimental change in stainless steel that happens when certain grades of stainless stay too hot for too long.

Here is a brief explanation that is as simple as I can make it...

Chromium molecules are dispersed rather evenly throughout 304L stainless and chromium is the main element that provides corrosion resistance in iron based stainless steels.

When stainless steel is heated up to around 800F,  carbon molecules will try to combine with the chromium molecules.

When that happens, they combine to form a chromium carbide.

 

This process of chromium combining with carbon is called carbide precipitation.

carbide precipitation causes the grain boundaries to be chromium depleted and therefore susceptible to corrosion.  

If the stainless is held between 800F-1200F for long enough, carbide precipitation will be extensive enough that the grain boundaries are compromised quickly when the part is put into service.

Think of a rock wall with mortar in between the rocks hold the wall together.

The mortar joints would be similar to grain boundaries in metal.

Then imagine there is such a thing as mortar termites and they begin eating the mortar joints.

Eventually the wall would crumble under strain.

When stainless steel that has been heated too much for too long is stressed, those chromium depleted grain boundaries cause tiny cracks.

This is known as stress corrosion cracking. (SCC for short).

SCC often shows up like a fine network of spider web like cracks. 

Stress corrosion cracking can be difficult or impossible to repair and often requires replacing the part or removing and replacing a large section of material.

That brings me to 321 stainless steel.

So what is 321 stainless steel and how is it different from other grades?

321 is very similar to 304L stainless in composition but 321 also contains a small percentage of titanium that acts as a stabilizer and ties up the carbon preventing it from combining with chromium. 

That’s why 321 is known as a stabilized grade of stainless steel.

Both motorsports and aerospace industries utilize 321 stainless steel because both industries have numerous components that operate at high temperatures.

321 stainless steel is commonly used for applications that require prolonged service in temperatures ranging from 800–1500°F.

For reference, at 800F, stainless steel is starting to look red hot and 1000 F is a very dull red heat.

So you can imagine the exhaust from a turbocharger operating at 900F might need to be made from 321 stainless to avoid carbide precipitation.