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Metals/stainless steel, MIG shield gas, Argon, Helium, CO2,

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QUESTION: Dear Sir,
        I have question regarding stainless steel 904L .For welding of SS 904L esp SMAW eletrode prescribed is E 385-16 which was matching rod.My doubt is if we do welding with E385-16 then after welding microstructure is complete austenite .
So how hot cracking will be controlled as
it is complete austenite structure and
also cr/ni equivalent comes less than 1.5
Ferrite no also is 0 ,but generally it should be in b/w 3-8 FN

ANSWER: I will respond using American designations.  I trust this will assist.

Common stainless steels such as 304 and 316 including the L versions, have an 18-8 ratio.  18% Chromium and 8% Nickel.  The 'L' indicates low Carbon content on the welding alloys.  This keeps the Carbon content less than 0.03%, and the microstructure is austenite.

In the USA, we mostly use 308L electrodes on 304 stainless.  For 316 stainless, be certain to use 316L electrodes.

Always use the prescribed stringer beads, not a weave bead, and hot cracking should not be an issue.

---------- FOLLOW-UP ----------

QUESTION: Dear sir,
      For welding stainless steel 904l we have been using ER 385 with MIG process for which i have seen Ar+30% He+2%CO2 has been prescribed.I want to confirm the same and generally where will get to know about selection of shielding gases like no hand books are there for selecting shielding gases

Answer
I have included this information on different shielding gases and where they are used.  It also includes some benefits and drawbacks as well.  Exactly which gas should be used in your process would require detailed conversation on the product you are making, the use of the product, and other manufacturing information - from a personal visit.  

I trust this will help clarify your information.

Why Do We Use Shielding Gases?

The primary purpose of the shielding gas is to protect the molten weld metal and the heat affected zone from contact with the atmosphere.  This is necessary because most metals when heated to their melting point in air, exhibit a strong tendency to form oxides.  Oxygen will also react with Carbon in molten steel to form Carbon Monoxide and Carbon Dioxide.  These reaction products result in trapped slag, porosity, and weld metal embrittlement.  

The air around us by volume is made up of 78% Nitrogen, 21% Oxygen, 0.94% Argon and 0.04% CO2 and other gases.  The air also contains a certain amount of water depending upon the humidity.  Of all the elements in the air, the two which cause the most welding difficulty, are Oxygen  and Hydrogen from the breakdown of moisture in the air.  

Inert gases are used for shielding and consist of very stable atoms that do not readily react with other elements.  There are only 6 such elements in nature with this stability and all exists as a gas.  

They are Argon, Helium, Krypton, Neon, Radon and Xenon.  Of the six, only Argon and Helium can be obtained in sufficient quantities at an economical cost.  Carbon Dioxide is also used extensively, although it is a reactive and not an inert gas.

Argon

Argon gives the most stabile arc control of any shielding gas. 100% Argon is only used when MIG welding Aluminum, Titanium and Copper.  Straight Argon used on steel, produces undercut and lack of fusion in the root of the weld deposit. For this reason it is used in gas blends such as 75% argon/25% CO2, commonly called 75/25.     Argon is ten times heavier than air.

Argon ����2

Mixing the penetration of carbon dioxide with the arc stability and low spatter of Argon has created a blend combining the best of both gas characteristics and a lower cost to produce than argon alone.  Argon/CO2 mixtures vary by percentage of each, and are used for welding mild and low alloy steels.  The Argon/CO2 blends can vary any where from 70% Argon to 98% Argon.

Argon plus Oxygen blends

When small amount of Oxygen in 1%, 2%, or up to 10% is added to Argon, it produces a broad, deep penetrating weld deposit.  This mix eliminates undercut and improves bead contour.  These blends are very common for welding Carbon and alloy steels and some stainless steels.

Carbon Dioxide

Carbon Dioxide is given off during any burning process and is a by-product of manufacturing ammonia and fermentation of alcohol, where it is almost 100% pure.  Carbon dioxide is known for its broad, deep penetration weld pattern, eliminating problems of lack of fusion and incomplete penetration.  It is also the least costly of the shielding gases.  

The chief drawback of Carbon Dioxide is the tendency for the arc to be somewhat violent.  This can lead to spatter problems, particularly on thin metals.  Carbon Dioxide is primarily used on mild steels although it is also mixed in other inert gas blends.

Helium

Helium is lighter than air and has high thermal conductivity.  The helium will expand in the arc, reducing the arc density.  This means some arc energy is lost in the arc itself and is not transmitted to the work.  The result is a broader bead with shallower penetration than Argon.  Because of high cost to manufacture, helium is used primarily for nonferrous welding aluminum, copper and magnesium due to the much higher heat levels possible.  

Argon-Helium-CO2

Commonly referred to in the USA as Tri-Mix, it is primarily used for welding 300 series stainless steels to control weld bead profile.  Tri-Mix produces a nearly flat bead vs. a typical covex bead, a requirement necessary in the food and drug industries.  These blends are used extensively in welding stainless steel pipe.

I hope this information has been helpful.

Rich Fercy
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Rich Fercy

Expertise

Specializing in wear resistant, industrial and construction steels. Also extensive background in welding cast iron, stainless steel and nickel based steels. Welding problems except code issues.

Experience

Since 1993 I have worked as a technical salesman in wear resistant steel applications and specialty welding solutions for Wisconsin and Northern Michigan.

Organizations
AFS MOTAG

Publications
Pit and Quarry Magazine, Jan 2014 - 'Wear Steel'. How to select the longest lasting wear resistant steel before writing a purchase order. www.pitandquarry.com/wear-steel/ Portable Plant & Equipment, September 2013 - 'Wear Steel'. Sister magazine to Pit and Quarry - same article. Recycling Today Magazine, August 2009 - "Wear On". Article compares Manganese Steel to Alloy Steel replacement shredder repair parts. Author of "40 Cast Iron Welding Secrets That Welding Shops Don't Want You To Know". Website: www.WearAnswers.com

Education/Credentials
Guest instructor at Fox Valley Tech College, Appleton WI, welding program. MSHA certified welding safety instructor.

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