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SS cutting and SS interpass temperature

SS cutting and SS interpass temperature

Postby Sanjay N Variya » Wed Nov 26, 2014 5:14 am

Dear all,
Please explain me why it isn't permitted gas cutting of stainless steel and why interpass temperature of stainless steel shall be 175 deg C and not 250 - 275 deg C as it is for carbon and alloy steels?
Sanjay N Variya
 
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Joined: Sun Sep 28, 2014 7:06 am

Re: SS cutting and SS interpass temperature

Postby Rockweld » Fri Nov 28, 2014 6:16 pm

Hello Sanjay,
To answer your question specifically it would be important to know what stainless steel you have been working with. Martensitic, Ferritic and Austenitic stainless steels all have different properties, chromium content etc. But to attempt a quick reply for you I think the answer is in what actually takes place within the Oxy-fuel cutting process of ferrous metals.
The process uses a unique torch with two primary functions. The first is to pre-heat and maintain an ignition temperature, above 1600 degrees F (870 degrees C.) The second process is the delivery of a stream of high-purity Oxygen. The actual “cutting” is the result of a rapidly advanced chemical reaction that takes place as the oxygen stream oxidizes the metal it contacts. The molten metal is then removed from the cutting line by the kinetic energy in the oxygen stream.
As alloys are added to the steel, the oxidation process becomes more difficult. Many of the high-melting oxides which are found in steels like aluminum and as your question referred to, chromium which is found in stainless steels, have melting points higher than iron oxides and as a result, may actually shield the material so that iron elements are not exposed to, or come in contact with, the oxygen stream. In fact, chromium oxide is extremely refractory and it melts at a much higher temperature than the stainless steels. As a result, the chemical reaction cannot take place and cutting is impossible.
I hope this helps you with the cutting challenge you face.
Rockweld
 
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Re: SS cutting and SS interpass temperature

Postby Jovi Zhu » Sat Nov 29, 2014 2:13 pm

Sanjay,

The oxyfuel gas cutting process is actually to "ignite" the preheated steel. Hence the relativelly high alloy content (especially Chrome) of stainless steels makes the ignition difficult. Plasma arc cutting (PAC) employing a constricted arc and removing molten metal with a high-velocity jet of ionized gas is usually used for cutting of steel with relatively high alloy level.

Generally ( I assume you are dealing with austenitic SS), interpass temperature (heat input) control is important to avoid "sensitization" and warping. The anti-corrosion ability of the joint will be impaired by excessive heat input.

Hope this helps,

Jovi
Jovi Zhu
 
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Re: SS cutting and SS interpass temperature

Postby Sanjay N Variya » Sun Nov 30, 2014 4:02 pm

Thanks Rockweld And Jovi for your best clarification...

I have cleared cutting reason of SS... But confusion about interpass... Plz explain briefly what will happen if interpass is 250C or more?
Sanjay N Variya
 
Posts: 7
Joined: Sun Sep 28, 2014 7:06 am

Re: SS cutting and SS interpass temperature

Postby Jovi Zhu » Mon Dec 01, 2014 12:13 pm

Sanjay,

The interpass temperature control is to limit the welding heat input to the joint. When previously deposited weld passes are not allowed to cool down below certain temperature (maximum required interpass temperature) before the subsequent passes are deposited, the welding heat will be accumulated hence result in too much heat input and expose the weld metal & HAZ into a too high temperature range for a certain period of time.

For common carbon and alloy steel, too much heat input will impair the impact toughness of the joint. If you have ASME IX code book at hand you will find all heat input related variables are listed as "supplementary essential" when toughness testing is required.

In addition to the impaired toughness, the precipitation of chromium carbides occurs along the grain boundaries (sensitization) when austenitic (300 series) and ferritic/martensitic (400 series) stainless steels are exposed for significant periods of time in the temperature range of about 1,000°F to 1,550°F. Because the grain boundary precipitation of the chromium carbides depletes the material immediately adjacent to the precipitates of chromium, the material can be highly susceptible to intergranular corrosion when exposed to corrosive aqueous environments.

Generally, it a good practice to weld stainless steels with low heat input. Oh I forget to mention that stainless steel, especially austenitc, conduct heat much more slowly than carbon/alloy steels.

Hope the above helps.

Jovi
Jovi Zhu
 
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Joined: Mon Aug 12, 2013 8:45 am

Re: SS cutting and SS interpass temperature

Postby Sanjay N Variya » Thu Dec 04, 2014 3:38 pm

Thanks Jovi, For your best clarification....
Sanjay N Variya
 
Posts: 7
Joined: Sun Sep 28, 2014 7:06 am


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