Cracks - H2 / HICC / Delayed / Underbead

The purpose to this article is to elaborate root causes contributing emergence of cracks in HAZ in C- Mn Steel & to provide insight on remedial action. This article is based on actual problems faced during Yard fabrication of Process modules.

Heat Affected Zone Cracking / Hydrogen Induced Cold Cracking (HICC):- HAZ cracking is characterized by separation of Weld metal & Parent metal that occurs immediately adjacent to weld bead. Although it is related to the welding process, the crack occurs in the parent material.

This type of cracks often originates from sub-surface, under or near to weld or HAZ hence known as “Under bead cracking” or “Toe Cracking”

As the cracking occurs after the steel has cooled down below 200 Deg. C so called as “Cold Cracking” & because the cracking associated with Hydrogen so referred as “Hydrogen Induced Cold Cracking”    

These cracks occurred most probably after 48 hours of welding so also called as “Delayed cracking”

Hydrogen-induced cracking occurs after weld cooling (hence the term cold cracking) and is often delayed for many hours while atomic hydrogen diffuses to areas of high tensile stress.

At microstructural flaws in a tensile stress field, the hydrogen changes to its molecular form, causing cracking. Cracking may occur in the HAZ or weld metal, and it may be longitudinal or transverse (Fig. 1).

Fig. 1 Hydrogen cracks originating in the HAZ and weld metal.

Mechanism of Hydrogen Cracking – The occurrence of cracking depends on number of factors like composition of steel, Welding procedure, welding consumables & the stress involved. If the cooling rate associated with welding is too rapid excessive hardening may occur in the heat affected zone. If sufficient hydrogen is present in the weld then the hardened zone may crack spontaneously under the influence of residual stress after the weld has cooled down to near ambient temperature.  

• H2 can be absorb in the welding arc from many sources including moisture on members, from air, from welding consumables covering, long or unstable arc,
• When weld metal is relatively hot (> 200 Deg. Cent.) H2 atoms can diffuse through the weld & HAZ quite rapidly & escape in to the atmosphere.
•  As the weld metal cools below Lower critical temperature (LCT) the weld metal transforms in to α ferrite / Pearlite that has far less solubility for H2. At this point H2 will tend to move in to HAZ where Austenitic phase is still retained.
• If the HAZ has high hardenability then transformation of HAZ from Austinite to Martensite structure takes place which offers less solubility for H2.
• If the cooling rate is rapid this H2 atoms get trapped in the HAZ. This will result in expulsion of H2 in Hard & Brittle micro structure.
• Cracks may occur from the areas of high stress concentration such as weld toes & generally move through hardened HAZ.

See below video to have an idea how hydrogen introduced to weld & leads to cracking of steel in the influence of other contributing factors.......

Factors influencing susceptibility to cracking in HAZ:- In order to occurred HAZ cracking following factors must be present independently or in an interacted manner, 

• Presence of Hydrogen in Weld Metal
• Presence of stresses (Stress Level)
• Susceptible micro structure {A brittle martensitic micro-structure produced by rapid cooling in HAZ area >350 HV Hardness (Composition of Steel) }
• Temperature of Weldment < 300 °C (Cooling Rate)
• Heat Input

Presence of Hydrogen in Weld Metal - Hydrogen is generally introduced into the weld area during welding. The sources are:

• Moisture in the covering of SMAW electrodes or in the core of FCAW electrodes,
• Hydrogen containing lubricants left on the surface of wire electrodes
• Hydrogen containing compounds or residues left on the surface to be welded (These can be Grease, Oil, Paint, Rust)
• High moisture in the shielding gas.
• Aspiration of moisture laden air in to weld area.
• The principle source of hydrogen is moisture contained in the flux (covering in SMAW electrodes) or the flux in core wires (FCAW).

The most effective means of avoiding hydrogen cracking is to reduce the amount hydrogen generated by the consumable i.e. by using a low hydrogen welding process.

Have a look on below video for Role of Hydrogen in cracking of steel.

Stress Level  – Cracks are more likely to initiate at regions of stress concentration, particularly at the toe & root of the weld.

The stresses generated across the welded joint as it contracts will be greatly influenced by external restraint, material thickness, and joint geometry & fit-up. Poor fit- up in filet welds increases the risk of cracking. The degree of restraint acting on a joint will generally increases as welding progresses.

There are always tensile stresses acting on weld because there are always residual stresses from welding. 

Magnitude of tensile stresses is mainly depends on thickness of component being welded, Heat input & Joint type.

The only practical way of reducing the influence of residual stresses could be by,

• Avoiding stress concentration due to poor fit- up,
• Avoiding poor weld bead profile ( sharp weld toes, Under cuts),
• Applying stress relief heat treatment / post heating after welding,
• Keeping weld metal volume as low as possible.

Parent Metal Composition – This will have a major influence on hardenability, with high cooling rates, the risk of forming a hard brittle structure in the HAZ. Hardenability of a material is usually expressed in terms of its carbon content or when other elements are taken in to account then its carbon equivalent (CE) value,   

Susceptible Micro structure - A Susceptible HAZ Micro structure is one that contains a relatively high proportion of hard brittle phases of steel, particularly Martensite.

From a metallurgical perspective, the carbon equivalent can be related to the development of hydrogen-sensitive microstructures. That is, as the carbon equivalent increases, microstructures are evolved during cooling through the transformation temperature range that are increasingly more susceptible to hydrogen-induced cracking. At high carbon equivalent values, martensitic structures can be expected.

The HAZ hardness is a good indicator of susceptibility when it exceeds a certain value. For C & C- Mn steel this value is 350 HV & Susceptibility to Hydrogen cracking increases with increase in Hardness above this value.

Hardness of an HAZ – Hardness of HAZ is influence by Chemical Composition of Steel and Cooling Rate of HAZ after each weld run.

Higher the Carbon equivalent (CEV) of a steel the greater the susceptibility to HAZ hardening, hence greater the susceptibility to HAZ cracking. Generally, steel with a CE value of < 0.4 are not susceptible to HAZ hydrogen cracking, as long as low hydrogen welding consumables or processes are used

Preheat temperature – Preheating is the process applied to raise the temperature of the parent metal before welding. It is used for the following main reasons:

• Removes moisture from weld,
• Preheating slower down cooling rate of the weld & HAZ, potentially resulting in softer weld metal and heat affected zone microstructures with a greater resistance to fabrication hydrogen cracking.
• The slower cooling rate encourages hydrogen to diffuse out from the weld area by extending the time period over which (particularly the time at temperatures above approximately 100°C) temperatures hydrogen diffusion rates are significantly higher than at ambient temperature. This reduction in hydrogen reduces the risk of cracking.
• Improves overall fusion characteristics during welding,
• Ensures more uniform expansion & contraction of material & thus lowering the level of Residual stresses.

The preheating temperature to be used can be obtained from below figure by reading the preheat line immediately above or to the left of the coordinated point for heat input and combined thickness at defined Ceq.

Cooling Rate: - Most structural carbon and low-alloy steels that may be susceptible to hydrogen-induced cracking transform from austenite region during cooling through the temperature range of 800 to 500 °C.

The length of time a steel spends in this temperature range during cooling will establish its microstructure and hence its cracking sensitivity. This time segment is generally referred to as t 8/5 (in seconds).

If the t8/5 time (cooling time from 800 °C to 500 °C) associated with welding is too short, excessive hardening can occur in the heat affected zone.

When the hydrogen level in the weld is above a critical level then the hardened zone can crack spontaneously under the influence of residual stress after the weld has cooled to near ambient temperature.

Faster the cooling rates after welding, greater the tendency for HAZ hardening.

Heat input and the cooling rate are directly related. With high heat input the weld cools slowly, and with low heat input, it cools quickly. For the microstructure of the heat-affected zone (HAZ) of a welded joint, the most crucial thing is the cooling time from +800°C to +500°C,

Procedures for avoiding hydrogen cracking, as well as selecting cooling times through the transformation temperature range to avoid hardened and susceptible microstructures, may involve controlling cooling in the lower temperature part of the thermal cycle, typically from 300 °C to 100 °C, thereby beneficially influencing the evolution of hydrogen from the welded joint. In particular, this can be achieved by the application of a post-heat on completion of welding which is typically a maintenance of the preheat temperature.

Parent Material Thickness :– Material thickness will influence the cooling rate, with increasing job thickness,

1) The welding cooling rate increases (welding cooling time, t8/5 decreases) & thus weld hardenability is increased,

2) The welding cooling time to 100 °C, t100 decreases & thus opportunity of effusion of diffusible hydrogen from weld metal decreases,

3) And as the weld pass increases the amount of hydrogen accumulated in weld metal is raised.

The combined thickness of the joint i.e. the sum of the thicknesses of material meeting at the joint line, will determine, together with the joint geometry, the cooling rate of the HAZ & its hardness. 

Post-heating - When there is a higher risk of cold cracking, hydrogen release should be accelerated by either maintaining the minimum inter pass temperature or raising the temperature to 200 °C to 300 °C immediately after welding and before the weld region cools to below the minimum inter pass temperature. The duration of post-heating should be at least 2 hr and is a function of the thickness. Large thicknesses require temperatures at the upper end of the stated range as well as prolonged post-heating times

Heat Input : - The heat input to the material from the welding process, together with the material thickness and preheat temperature, will determine the thermal cycle and the resulting microstructure and hardness of both the HAZ and the weld metal.

Increasing the heat input will reduce the hardness level, and therefore reduce the risk of HAZ cracking. However, as the diffusion distance for the escape of hydrogen from a weld bead increases with increasing heat input, the risk of weld metal cracking is increased.

Detection and remedial action:- As hydrogen cracks are often very fine and may be sub-surface, they can be difficult to detect. Surface-breaking hydrogen cracks can be readily detected using visual examination, liquid penetrant or magnetic particle testing techniques. Internal cracks require ultrasonic or radiographic examination techniques. Ultrasonic examination is preferred, as radiography is restricted to detecting relatively wide cracks that are parallel to the beam. As the formation of cracks may be delayed for many hours after completion of welding, the delay time before NDT inspection should be observed.

A cracked component should be repaired by removing the cracks with a safety margin of approximately 5mm beyond the visible ends of the crack. The excavation to be re- inspected with NDT & re-welded. To make sure that cracking will not re-occur, Procedure qualification test shall be carried out on higher CEq material having highest section thickness with extra low hydrogen welding consumables at Preheating of 200 °C & post heating of 230 - 400 °C for 2 - 4 hours. Welding consumables used shall be of Hermetically sealed Vacuum Packed as these welding consumables produces god quality welds due to their Low Moisture Absorption (LMA) characteristics & eliminates human interface of Drying / Baking / Holding in ovens.  

Look at the final view of Oil Stabilisation Module after installation at field.....