Shielding gases FAQ

 

Description

Shielding gases FAQ

Are you using the right gases correctly?

 

Here are a few of the many thousands of questions BOC engineers are asked every year. Some of you will know the answer but for others, the answer may not be what you were expecting.

 

 

Can my gases have separated in the cylinder?

 

Gases don’t separate in a cylinder. In a cylinder, the gas molecules are constantly in motion and this ensures total mixing. If gases didn’t stay mixed, air would have separated into oxygen and nitrogen by now!

 

 

Why am I getting holes in my welds?

 

Holes (porosity) are usually caused by gas entrapment inside the cooling weld metal. While gases such as nitrogen are one of the main causes of porosity, other sources such as water, oil and grease on the material can be as much of a problem.
The main causes of porosity are:

  • too high or too low a flow of shielding gas – too high and air is entrained into the shield; too low and the gas can’t protect the cooling weld metal from the atmosphere
  • poor welder technique – too long a stick-out or bad torch angle
  • incorrect choice of shielding gas – shielding gases containing hydrogen and/or nitrogen are beneficial for some materials but can cause porosity in others
  • poorly maintained equipment
  • if hose fittings are not tightened
  • or if there are gas leaks in the power source or torch, air can be entrained into the shielding gas.

Also some types of hose are permeable and can allow moisture to enter the shielding gas. Surface contamination – oil, grease, water and other contamination on the welded component can add hydrogen into the weld metal.

 

This is not an exhaustive list but most causes of porosity are due to poor housekeeping and/or poor welding procedures.

 

 

Why can I not use pure argon for GMA welding steels?
 

While it is possible to GMA-weld steels with pure argon, the arc produced is very unstable and erratic, and the resultant weld will have a lot of spatter and an unsatisfactory penetration profile.

 

When GMA welding steels, a small amount of oxidising gas (either carbon dioxide or oxygen) is needed to help to stabilise the arc and produce sound welds.

 

 

Why am I getting a lot of spatter on my welds?


There are several causes of spatter, but the most common are:

  • using unstable welding conditions – incorrect voltage for a given welding current
  • poor welder technique – too long a stick-out or bad torch angle
  • surface contamination on component – oil, grease, moisture
  • surface coatings such as paint and zinc galvanising
  • using carbon dioxide as the shielding gas – mixed gases are more stable and produce less spatter

Training the welder to set good welding conditions and clean the component properly can eliminate many of the problems.

 

 

I get cracking when welding stainless steels. Why?


There are two main types of cracking in stainless steels: ‘hot cracking’ and ‘cold cracking’.

 

Hot cracking, properly called ‘solidification cracking’, tends to be a problem in austenitic stainless steels. It is called ‘hot cracking’ as it tends to occur immediately after welding while the weld is still hot. Weld-metal solidification cracking is more likely in fully austenitic structures which are more crack-sensitive than those containing a small amount of ferrite. The best way to prevent cracking is to choose a consumable which has a high enough ferrite content to ensure that the weld metal does not crack.

 

Cold cracking, properly called ‘hydrogen cracking’, occurs in welds that are intolerant of hydrogen (e.g. martensitic stainless steels). Hydrogen dissolves in the weld metal while it is molten then after solidification it diffuses to small defects in the weld and hydrogen gas forms, building up in pressure as the weld cools. Then, when the pressure is sufficiently high and the weld is cool and more brittle, this internal pressure can cause the weld to crack. This may not occur until many hours after welding.

 

 

What causes the sooty deposit when welding aluminium?


This sooty deposit is not soot (carbon) at all, but a form of aluminium oxide.

 

When welding occurs, some of the parent material and filler wire is volatilised by the welding arc. As this fine metal vapour leaves the area covered by the shielding gas, it reacts with air, forming aluminium oxide that condenses on the component being welded. The higher the welding current used, the greater the amount of oxide produced.

 

It is not always possible to eliminate this problem but altering the torch angle and ensuring correct shielding gas coverage can minimise the effect. Also, if the weld is cleaned immediately after welding, the oxide is much easier to remove than if it is left until the weld is cold. The use of an Alushield® shielding gas will also help reduce the coverage of aluminium oxide.