TWI Knowledge Summary

Arc weldability of alloy steels

by Adrienne Barnes

Cracking

Alloy steels are potentially susceptible to the following types of cracking:

Hydrogen cracking
Solidification cracking
Reheat cracking
Liquation cracking

With knowledge of the factors that control these cracking mechanisms, it is generally possible for weld procedures to be developed for most modern alloy steels to allow crack-free weldments to be produced by arc welding. More details on each of these cracking mechanisms can be found in Fabrication cracking mechanisms in ferritic steels - A guide to best practice

Mechanical properties

In the majority of cases, elevated temperature strength and oxidation behaviour are the key areas of concern. Joint strength can generally be readily achieved through the use of appropriate welding consumables although the long term creep performance can be reduced relative to base material (by up to ~20%), particularly in higher alloy grades, by the development of a creep-weak or 'soft' zone in the outer HAZ, known as the Type IV zone.

These materials are generally used in the PWHT condition and the toughness of both HAZ or weld metal is usually satisfactory. For low alloy steels, HAZ toughness can be improved by ensuring a sufficient level of alloying to give transformation to low carbon martensite rather than bainite, and in this respect Ni-additions can be beneficial.

However, depending on the impurity content, low alloy steels may be susceptible to temper embrittlement if cooled slowly through the 350-600°C temperature range, either during PWHT or, more commonly following elevated temperature service. Susceptibility to temper embrittlement can be minimised through careful control of impurity levels in base material and welding consumables at the time of procurement. For more information, look at the FAQ - What is temper embrittlement and how can it be controlled?

Service degradation

Welds in alloy steels may be subject to temper embrittlement (as previously discussed), hydrogen embrittlement, hydrogen attack and creep damage.

During elevated temperature hydrogen service, gaseous hydrogen can enter steel and, if the partial pressure and temperature are below those required to achieve hydrogen attack, no problems will be encountered at elevated temperature. If allowed to cool to near ambient temperature, for example during shut down, hydrogen embrittlement can occur which may lead to cracking. If the temperature and partial pressure of hydrogen are high, hydrogen attack may occur. The hydrogen absorbed into the steel, combines with carbon in the steel to form methane; this may lead to decarburisation and the formation of voids or microcracks and blistering can develop.

If subjected to an applied load at elevated temperature (within the creep regime) for extended periods of time, creep damage/deformation may occur. Creep occurs by a combination of dislocation movement, grain boundary sliding and diffusion. Creep damage can vary in severity from a coarsening of carbides and the formation of isolated cavities to orientated cavities through to microcracks and macrocracks. Where a material operates within the creep regime regular inspection, including dimensional measurements and microstructural replication, is required to allow the material condition and residual life to be assessed.

Further information

Related content on weldability can be found in the items listed below:

What is weldability
Arc weldability of micro alloyed steels
Job knowledge for welders: Weldability of materials - carbon manganese and low alloy steels

Arc weldability of dissimilar metals
Fabrication cracking mechanisms in ferritic steels - A guide to best practice

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