What is 'Weldability'?

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What is 'Weldability'?

مُساهمة من طرف Admin في الخميس سبتمبر 13, 2007 11:51 pm

[TWI Knowledge Summary
What is 'Weldability'?
by Richard Pargeter
To many people 'weldability' is simply a measure of how easy it is to make a weld in a particular material without cracks. If it is easy to avoid cracking, the material is deemed 'weldable'. For a weld to be truly successful, however, it is also necessary for it to have adequate mechanical properties, and to be able to withstand degradation in service (eg corrosion damage). Thus, weldability is a measure of how easy it is to:

Obtain crack free welds
Achieve adequate mechanical properties
Produce welds resistant to service degradation.
Weldability is not a fixed parameter for a given material, but will depend on joint details, service requirements, and welding processes and facilities available. This variability in weldability is illustrated in the following examples:

Example 1
Which of these two C-Mn steels is most weldable?

Steel 1 Steel 2
C 0.16 0.19
S 0.027 <0.002
P 0.011 0.021
Si 0.20 0.28
Mn 0.61 1.38
Ni 0.03 0.01
Cr 0.02 0.02
Mo <0.01 < 0.005
V <0.01 <0.01
Cu 0.03 0.005
Nb <0.005 0.024
Ti <0.01 0.002
Al <0.001 0.047
CE IIW 0.27 0.43
Pcm 0.20 0.27
CEN 0.27 0.43

The answer clearly depends on which type of cracking is of most concern:

Low restraint fillet onto thick steel
- Hydrogen crack, steel 1 more weldable

Restrained high dilution MIG nozzle weld
- solidification crack, steel 2 more weldable

Full penetration highly restrained T butt
- lamellar tearing, steel 2 more weldable.
Example 2
Which of these materials is most weldable? (welding a fairly thin walled (~3mm) pipe)

Commercially pure titanium
316 L austenitic stainless steel
22% Cr duplex stainless steel
6% Mo high alloy austenitic stainless steel
The answer will depend on an individual's experience, and available facilities.

The titanium expert knows that it is one of the easiest materials to weld - but he is very familiar with very good back purges, and the use of a trailing shield.

The expert in austenitic stainless steel would see this level of control to be very difficult. He knows to watch out for solidification cracking, and is careful to check the penetration characteristics of each cast, and does not consider that these pose a significant risk.

An expert in duplex stainless steels will tell you that it is much easier to weld than austenitic stainless steel, because there is no real risk of solidification cracking, and less of a variable penetration problem. But now, you generally need a filler.

High alloy austenitic steel is similar to duplex, expect that with a Ni based filler there is a risk of microfissuring.

Example 3
Consider Example 2, but now add that the weld will be operating in an acid, chloride containing environment at about 30°C. Had the concern been purely about freedom from cracking, then duplex and titanium would have been on an equal footing, with the high alloy austenitic being the least weldable because of the risk of solidification cracking. Now, however, the duplex stainless steel becomes more of a problem, as it becomes necessary to work within quite a narrow heat input window. It can be difficult to pass procedure qualification tests involving corrosion tests with duplex stainless steels.

Example 4
Consider examples 2 and 3, but now add a toughness requirement. Now titanium is not so weldable, as near perfect shielding is necessary to avoid toughness degradation.

Example 5
Is AISI 4130 weldable?

The composition range for AISI 4130 is:

C 0.27-0.34
S <0.040
P <0.035
Si 0.15-0.35
Mn 0.35-0.60
Cr 0.80-1.15
Mo 0.15-0.25

It is not possible to answer this question without knowing the intended service. The answer would be different for a gear component, to operate in a warm oil bath, and a piece of wellhead equipment to carry sour gas]


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