ÒõAPPµ¼º½

In this article we will discuss the items that assist in producing Hydrogen Induced Cold Cracking (HICC) in welds.

Hydrogen Induced Cold Cracking occurs in ferritic steel weld zones, it can occur while the weld area cools and it can also be delayed for a period of time upon completion of welding.Ìý

HICC can occur both in the weld metal and in the Heat Affected Zone (HAZ). This fact distinguishes HICC from solidification cracking that was recently discussed in an article by Mr. Nairn Barnes. Solidification cracking is a weld metal phenomenon only and cannot occur in the HAZ. The other distinguishing feature of HICC is that it manifests itself at low temperatures whereas solidification cracking, as the name implies, occurs at relatively high temperatures.

So, what are the major factors that influence hydrogen induced cold cracking (HICC) in weldments. There are four factors and they are:

1. The hydrogen content of the weld zone
2. The susceptibility of the weld or of HAZ to hydrogen embrittlement. This is related to the chemical composition and final microstructure that is formed. This is influenced by how fast the weld cools. (the cooling rate, otherwise understood as the quenching effect).Ìý
3. The stress level at the point of crack initiation
4. Temperature – cracking can only occur after the weld has cooled below about 150°C.Ìý

Let’s take a closer look at these four points above.Ìý

1. The hydrogen content of weld zone?

Where does hydrogen come from? It comes from:

• The welding process and/or the welding consumable will be a source of hydrogen and, thus, their selection needs to be correct in order to reduce the probability of HICC
• The water vapour in the air (humidity) or in the shielding gas
• From moisture on/in electrode coatings (e.g. SMAW) and grease on a bare wire (e.g. GTAW or GMAW)
• From any grease, paint or corrosion products such as scale on the plate or pipe to be welded.

Hydrogen, obtained from the sources shown above, is readily dissolved in molten weld metal as atomic hydrogen. The hydrogen will begin to diffuse out of the weld metal during cooling, escaping into the surrounding atmosphere and recombining as hydrogen gas or diffusing through the HAZ into the parent material.Ìý

2.0 The susceptibility of the weld or HAZ to hydrogen embrittlement.Ìý

This is related to the chemical composition and the resulting microstructure of the weld. This, in turn, is related to how fast the weld area cools (known as the cooling rate). The hardness formed is the key property and the higher the hardness the greater the probability of forming HICC, all other things being equal.

Certain steels, normally the higher strength variety, will be able to form harder microstructures on cooling than the nominally lower strength steels. The ability of steel to harden when it is cooled from high temperatures is called its hardenability.Ìý

There can be several different microstructures in a weld zone, those in the weld metal and those in the HAZ. The HAZ, in particular, has several different microstructures. This is due to the effect of heating and cooling being different across the zone, from FL (fusion line) to the extremity of the HAZ at PL (unaffected parent metal line) as shown in Figure 1.

Ìý

Figure 1 Macro showing Weld Metal and Heat Affected Zones

3.0 Stress at crack initiation area.Ìý

This is the local stress at the site where the crack wants to initiate and will be affected by such things as joint restraint and stress concentrators. HICC can result by the diffusion of hydrogen to a highly stressed, hardened part of the weldment.

Ìý

Figure 2 Illustrating a Poor Weld Toe Profile which is a Stress Concentrator

Figure 2 illustrates a sharp weld toe profile which is viewed as an area of stress concentration and this is a possible site for HICC to initiate.

4.0 Temperature

Cracking can only occur after the weld has cooled usually to temperatures at or near normal ambient, usually below about 150°C. Hence the use of the word cold in the cracking description.Ìý

In theory then, if we could use or operate all our welded ferritic steel equipment above 150 deg C, we would never see a hydrogen crack.Ìý

So now that we have illustrated the four factors that can lead to HICC we can consider what we can do to prevent, or reduce, the probability of its occurrence. This will be discussed in detail in a following article which will be published in the next few weeks.