Defects - hydrogen cracks in steels - identification

Preheating to avoid hydrogen cracking

Hydrogen cracking may also be called cold cracking or delayed cracking. The principal distinguishing feature of this type of crack is that it occurs in ferritic steels, most often immediately on welding or a short time after welding.

In this issue, the characteristic features and principal causes of hydrogen cracks are described.

Identification

Visual appearance Hydrogen cracks can usually be distinguished due to the following characteristics:

In C-Mn steels, the crack will normally originate in the “Heat Affected Zone” (HAZ), but may extend into the weld metal (Fig 1 below).

Cracks can also occur in the weld bead, normally transverse to the welding direction at an angle of 45° to the weld surface. They follow a jagged path, but may be non-branching.

In low alloy steels, the cracks can be transverse to the weld, perpendicular to the weld surface, but are non-branching, and essentially planar.

Fig. 1 Hydrogen cracks originating in the HAZ and weld metal. (Note that the type of cracks shown would not be expected to form in the same weldment.)

On breaking open the weld (prior to any heat treatment), the surface of the cracks will normally not be oxidized, even if they are surface breaking, indicating they were formed when the weld was at or near ambient temperature. A slight blue tinge may be seen from the effects of preheating or welding heat.

Metallography

Cracks which originate in the HAZ are usually associated with the coarse grain region, (Fig 2). The cracks can be intergranular, Tran granular or a mixture. Intergranular cracks are more likely to occur in the harder HAZ structures formed in low alloy and high carbon steels. Tran granular cracking is more often found in C-Mn steel structures.

In fillet welds, cracks in the HAZ are usually associated with the weld root and parallel to the weld. In butt welds, the HAZ cracks are normally oriented parallel to the weld bead.

Fig. 2 Crack along the coarse grain structure in the HAZ

Causes

There are three factors which combine to cause cracking:

Hydrogen generated by the welding process

A hard brittle structure which is susceptible to cracking

Tensile stresses acting on the welded joint

Cracking usually occurs at temperatures at or near normal ambient. It is caused by the diffusion of hydrogen to the highly stressed, hardened part of the weldment.

In C-Mn steels, because there is a greater risk of forming a brittle microstructure in the HAZ, most of the hydrogen cracks are to be found in the parent metal. With the correct choice of electrodes, the weld metal will have a lower carbon content than the parent metal and, hence, a lower carbon equivalent (CE). However, transverse weld metal cracks can occur, especially when welding thick section components; the risk of cracking is increased if the weld metal carbon content exceeds that of the parent steel.

In low alloy steels, as the weld metal structure is more susceptible than the HAZ, cracking may be found in the weld bead.

The main factors which influence the risk of cracking are:

Weld metal hydrogen.

Parent material composition.

Parent material thickness.

Stresses acting on the weld during welding or imposed (shortly) after welding.

Heat input.

Weld metal hydrogen content

The principal source of hydrogen is moisture contained in the flux, i.e. the coating of MMA electrodes, the flux in cored wires and the flux used in submerged arc welding. The amount of hydrogen generated is influenced by the electrode type. Basic electrodes normally generate less hydrogen than rutile and cellulosic electrodes.

It is important to note that there can be other significant sources of hydrogen, e.g. from the material, where processing or service history has left the steel with a significant level of hydrogen or moisture from the atmosphere. Hydrogen may also be derived from the surface of the material or the consumable.

Sources of hydrogen will include:

Oil, grease and dirt.

Rust.

Paint and coatings.

Cleaning fluids.

Parent metal composition

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

The higher the CE value, the greater the risk of hydrogen cracking. Generally, steels 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.

Parent material thickness

Material thickness will influence the cooling rate and therefore the hardness level, the microstructure produced in the HAZ and the level of hydrogen retained in the weld.

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 and its hardness. Consequently, as shown in Fig. 3, a fillet weld is likely to have a greater risk than a butt weld in the same material thickness.

Fig.3 Combined thickness measurements for butt and fillet joints

Stresses acting on the weld

Cracks are more likely to initiate at regions of stress concentration, particularly at the toe and 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 and fit-up. Poor fit-up (excessive root gap) in fillet welds markedly increases the risk of cracking. The degree of restraint acting on a joint will generally increase as welding progresses, due to the increase in stiffness of the fabrication.

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.

Heat input per unit length is calculated by multiplying the arc energy by a thermal efficiency factor, according to the following formula:

V = arc voltage (V) A = welding current (A) S = welding speed (mm/min) k = thermal efficiency factor

In calculating heat input, the thermal efficiency must be taken into consideration. The thermal efficiency factors given in EN 1011-1: 1998 for the principal arc welding processes are:

Submerged arc (single wire)

1.0

MMA 0.8

MIG/MAG and flux cored wire 0.8

TIG and plasma 0.6

In MMA welding, heat input is normally controlled by means of the run-out length from each electrode, which is proportional to the heat input. As the run-out length is the length of weld deposited from one electrode, it will depend upon the welding technique, e.g. weave width /dwell.

Mechanisms

The mechanism starts with lone hydrogen atoms diffusing through the metal. At high temperatures, the elevated solubility of hydrogen allows hydrogen to diffuse into the metal (or the hydrogen can diffuse in at a low temperature, assisted by a concentration gradient). When these hydrogen atoms re-combine in minuscule voids of the metal matrix to form hydrogen molecules, they create pressure from inside the cavity they are in. This pressure can increase to levels where the metal has reduced ductility and tensile strength up to the point where it cracks open (hydrogen induced cracking, or HIC). High-strength and low-alloy steels, nickel and titanium alloys are most susceptible. Austempered iron is also susceptible. Steel with an ultimate tensile strength of less than 1000 MPa or hardness of less than 30 HRC are not generally considered susceptible to hydrogen Embrittlement. Jewett Reports the results of tensile tests carried out on several structural metals under high-pressure molecular hydrogen environment. These tests have shown that austenitic stainless steels, aluminum (including alloys), copper (including alloys, e.g. beryllium copper) are not susceptible to hydrogen Embrittlement along with few other metals. For example of a severe Embrittlement measured by Jewett, the elongation at failure of 17-4PH precipitation hardened stainless steel was measured to drop from 17% to only 1.7% when smooth specimens were exposed to high-pressure hydrogen.

Hydrogen Embrittlement can occur during various manufacturing operations or operational use - anywhere that the metal comes into contact with atomic or molecular hydrogen. Processes that can lead to this include cathodic protection, phosphating, pickling, and electroplating. A special case is arc welding, in which the hydrogen is released from moisture (for example in the coating of the welding electrodes; to minimize this, special low-hydrogen electrodes are used for welding high-strength steels). Other mechanisms of introduction of hydrogen into metal are galvanic corrosion, chemical reactions of metal with acids, or with other chemicals (notably hydrogen sulfide in sulfide stress cracking, or SSC, a process of importance for the oil and gas industries).

Counteractions

If the metal has not yet started to crack, the condition can be reversed by removing the hydrogen source and causing the hydrogen within the metal to diffuse out, possibly at elevated temperatures. Susceptible alloys, after chemical or electrochemical treatments where hydrogen is produced, are often subjected to heat treatment to remove absorbed hydrogen. There is a 4-hour time limit for baking out entrapped hydrogen after acid treating the parts. This is the time between the end of acid exposure and the beginning of the heating cycle in the baking furnace. This per SAE AMS 2759/9 Section 3.3.3.1 which calls out the correct procedure for eliminating entrapped hydrogen.

In the case of welding, often pre- and post-heating the metal is applied to allow the hydrogen to diffuse out before it can cause any damage. This is specifically done with high-strength steels and low alloy steel such as the chrome/molybdenum/vanadium alloys. Due to the time needed to re-combine hydrogen atoms into the harmful hydrogen molecules, hydrogen cracking due to welding can occur over 24 hours after the welding operation is completed.

Products such as ferrosilicates can be used to treat surfaces normally subject to hydrogen Embrittlement in order to prevent it from taking place.

Related phenomena

If steel is exposed to hydrogen at high temperatures, hydrogen will diffuse into the alloy and combine with carbon to form tiny pockets of methane at internal surfaces like grain boundaries and voids. This methane does not diffuse out of the metal, and collects in the voids at high pressure and initiates cracks in the steel. This process is known as hydrogen attack and leads to decarburization of the steel and loss of strength.

Decarburization is also a selective leaching type of corrosion when carbon is preferentially removed from the surface of the alloy. One of the mechanisms is high temperature hydrogen attack, when hydrogen reacts with carbon and

carbides, resulting in loss of strength and ductility and formation of internal fissures.

Copper alloys which contain oxygen can be embrittled if exposed to hot hydrogen. The hydrogen diffuses through the copper and reacts with inclusions of Cu2O, forming H2O (water) which then forms pressurized bubbles at the grain boundaries. This process can cause the grains to literally be forced away from each other, and is known as steam Embrittlement (because steam is produced, and not because exposure to steam causes the problem).

Researched by Robert Mitchell 02 August 2010