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  • FAQ: What is the relationship between hardness, microstructure and toughness in steel heat affected zones?

What is the relationship between hardness, microstructure and toughness in steel heat affected zones?

   

There is no simple relationship between these three items, although they are related as indicated in BS EN 1011-2 Annex D (Reference 1) by a graphical example:

Fig. 1. An example of how impact energy is affected by the welding thermal cycle
Fig. 1. An example of how impact energy is affected by the welding thermal cycle

However, it is difficult to relate this then to hardness or microstructure.

The microstructure and hardness produced in any ferritic steel heat affected zone (HAZ) is essentially dependent upon:

  1. the cooling rate through the transformation temperature range of the steel in question.
  2. the composition and the hardenability of the steel, and
  3. the (prior austenite) grain size before transformation.

The cooling rate is governed by the heat supplied during welding, and the heat sink, which is a function of the initial temperature of the parts to be joined, their thickness and geometry. In arc welding, the heat supplied during welding is characterised by the heat input, which is defined as

faqjmn034e1.gif

(see What is the difference between heat input and arc energy?)

Control over cooling rate in a particular fabrication (where steel composition and geometry are fixed) is therefore achieved by varying heat input and preheat temperature.

The hardness generated in the steel at a given cooling rate is governed principally by its composition, and a useful way of describing this is to assess the total contribution to it of all the elements that are present. This is done by empirical formulae which define a carbon equivalent (CE) value and takes account of the important elements which are known to affect hardenability. One formula used is:

faqjmn034e2.gif

 

Its calculation and use are described in detail in What is the difference between the various Carbon Equivalent Formulae used in relation to hydrogen cracking?. The cooling rates which produce different microstructures of different hardnesses are established by laboratory studies of each steel type, using the cooling rates which the steel experiences during welding.

For carbon-manganese steels, a relationship between composition (CE value), cooling rate and microstructural hardness level has been established. This relationship has been used in constructing welding diagrams for these steels, in order to avoid fabrication hydrogen cracking of the heat affected zone. These diagrams are given in reference[2] , and also in the Preheat Toolkit.

Low toughness may be experienced in HAZs due to the presence of inherently brittle microstructures, including unusually coarse microstructures.

A low heat input leads to rapid cooling as the weld deposited is small in relation to the parent material and the parent material acts as a heat sink. The toughness can be low in microstructures that have arisen from rapid cooling rates. In general, very low heat inputs are to be avoided as they result in hard, crack susceptible microstructures with poor toughness.

A high heat input gives slower cooling and the grain size in the HAZ can become very coarse if the temperature is high enough to promote grain growth prior to transformation. Very large grain sizes can have poor toughness even when the microstructure is soft.

Many other factors also contribute to HAZ toughness, however, and neither hardness nor microstructure alone can be used as reliable indicators of toughness. See Local brittle zones in C-Mn steel multipass welds.

In TMCP steels, a limit is often placed on heat input to avoid undue softening in the weld region, (see Is there a restriction on the heat input that can be used for the welding of TMCP (Thermo-Mechanically Controlled Processed) steels - if so, why?)

Reference

  1. BS EN 1011-2:2001 Welding - Recommendations for welding of metallic materials. Arc welding of ferritic steels
  2. Bailey N, Coe F R, Gooch T G, Hart P H M, Jenkins N and Pargeter R J: 'Welding steels without hydrogen cracking' Second Edition, 1993, Abington Publishing.

Further information

See further information about Materials and Corrosion Management, or please contact us.   

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