316 SS Rusting yes Rust

Date 03-12-2008 03:49

It is quite possible that the heating process caused chromium carbide precipitation. Stainless steels have a protective layer of Chromium oxide on the surface. If the SS is heated between 400-800 C, some of the chromium binds with carbon and hence is not available to provide corrosion resistance; this happens especially near grain boundaries. Such SS can then rust. To avoid this problem try bending the brackets without heating. If that is not possible, you can heat the brackets (after bending) to 1000C for a 2-4 hours and rapidly cool them; this heat treating process breaks down the carbides and makes the Chromium free again. The heat treatment to 1000C can cause some annealing.

Did you have a different procedure to bend the brackets before this batch?

Girish

http://www.welding-consultant.com

By DaveBoyer (*****)

Date 03-12-2008 06:32

Girish is telling it like it is. There must be no chrome carbides, using 316L material would help. Other processes help too.

A problem is that the chrome oxide film can become depleted if the surface is not exposed to oxygen. This is common where a stainless part is mounted to a surface, and rust bleeds from the joint.

Electro-polishing removes the iron from the surface leaving the chrome & nickel.

There are chemical pickling processes that leave a passive chrome oxide surface as well.

By gwg

Date 03-14-2008 01:06

There is another possibility - surface contamination from the hot forming (bending) operation. Do the rust stains appear in a band or streaks.

By DaveBoyer (*****)

Date 03-14-2008 02:37

gwg is describing iron transfer, iron from bending jigs etc. rubbs off on the stainless. This can be removed by mechanical polishing, electro-polishing or chemical methods.

By Hugh Cunningham (*)

Date 06-04-2008 12:52

I believe what you are experiencing is simple heat tint. The heated area should be treated with a pickling and passivation paste after welding to remove the heat tint and re-establish the passive layer. Alternatively, you can grind the area, but please not that simple hand wire brushing is usually NOT sufficient to remove the heat tint ( a common misconception by cheapskate contractors trying to save a few pennies at the expense of quality!).

I agree that metal transfer can happen when the workpiece is in contact with carbon steel (chains, hooks, supports, etc.). This manifests itself as rusty scores and marks on the surface. Think what will happen if a carbon steel wire brush is used on the SS surface!

By cwi49cwe (*)

Date 09-20-2009 22:15

I agree, the break press added an iron contanination layer to the part at the time of bending , also after heat bending 304 and 316 it is a very common thing to drop them in a cold water bath, this stops the carbon precip. and restores the austenitic structure to the stainless, take them out of the water, power clean the break press area of contamination and sand blast the heat marks if they are a problem...