TWI Knowledge Summary

Arc weldability of micro alloyed steels

By Richard Pargeter

Weldability

Weldability encompasses the ability to make crack-free welds, the ability to achieve desired properties, and the ability to make welds which will survive service conditions. Please see What is weldability?

Cracking

Cracking problems of potential concern in microalloyed steels are:

Hydrogen cracking
Solidification cracking
Lamellar tearing
Liquation cracking

In most modern microalloyed steels, the composition gives good resistance to all these cracking mechanisms during arc welding although the risk of hydrogen and solidification cracks is still influenced by welding procedure. Parameters controlling solidification cracking in power beam welding are not fully defined at present.

More information in best practice guide: Fabrication cracking mechanisms in ferritic steels

Mechanical properties

Joint strength is generally easily achieved with the use of appropriate consumables. If the steel is thermomechanically processed some control over maximum heat input may be necessary to avoid excessive HAZ (heat affected zone) softening.

Where HAZ toughness is important, care is necessary to ensure that adequate levels are achieved. Low HAZ toughness can be experienced in Nb microalloyed steels (>0.02%Nb), welded at high heat input (800-500°C cooling time over ~25 seconds, equivalent to ~2.4kJ/mm in 15mm thick steel) and also, in steels with combined Nb and V additions in the PWHT (post weld heat treated) condition. Phosphorus levels over about 0.012% may also lead to low PWHT HAZ toughness. Toughness can also be degraded in outer (eg fusion line +5mm) HAZ regions. Ti additions can enhance HAZ toughness, but if not done correctly, coarse Ti containing particles can act as crack initiators.

Weld metal toughness can be affected by dilution, and there is a risk of toughness degradation in welds made with basic fluxes in steels with high Al contents giving weld metal Al:O over ~1, or in high oxygen welds which have picked up over ~0.02% Nb from parent plate. However, where high levels of toughness are required, basic fluxes should be used, in which normal levels of Nb dilution can be tolerated.

Service Degradation

Welds in microalloyed steels may be subject to strain ageing and stress corrosion cracking. The base toughness of most modern microalloyed steels is sufficient to ensure that strain ageing in service is not a concern.

For welds to be exposed to sour (H ₂S) containing environments the risk of sulphide stress corrosion cracking (SSCC) must be taken into account. Susceptibility to this form of cracking is generally controlled through hardness, and the usual limit of ~250HV is readily achievable in lower carbon equivalent grades.

For welds to be exposed to anhydrous ammonia, the risk of stress corrosion cracking is principally dependent on steel strength level, and cannot be limited by hardness control. For steels over 350MPa yield, stress relief will be necessary. Steels of over 460MPa yield are not recommended without strict environmental controls.

Most other forms of stress corrosion cracking (eg caustic cracking) are not strongly material dependent, and all microalloyed steels may be considered equally weldable (or unweldable) depending on the service environment. Post weld heat treatment can be beneficial for intermediate service conditions.

Further information

For related content items on weldability, please look at:

Job knowledge for welders: Weldability of materials - carbon manganese and low alloy steels . (Aug.1996)

Arc weldability of dissimilar metals

Fabrication cracking mechanisms in ferritic steels - a guide to best practice. Section 1. Introduction and Contents

You can use the Weldasearch literature database to supplement what you find in JoinIT.