If weld preparation is excellent and operator-induced defects (e.g., lack of penetration or fusion) are avoided, all the standard structural steels can be successfully welded. However, several steels may require special treatments to achieve a satisfactory joint. These treatments are not convenient in all cases.
The difficulty in producing satisfactory welded joints in some steels arises from the extremes of heating, cooling, and straining associated with the welding process, combined with microstructural changes and environmental interactions during welding. Some structural steels can’t tolerate these effects without joint cracking occurring. The various types of cracking which can occur and the remedial measures which can be taken are discussed below.
Weld Metal Solidification Cracking
Solidification of the molten weld pool occurs by the growth of crystals away from the fusion boundary and towards the center of the weld pool until there is no remaining liquid. In crystal growth, solute, and impurity elements are pushed ahead of the growing interface. This process is not significant until the final stages of solidification, when the ever-increasing crystals interlock at the center of the weld.
The high concentration of solute and impurity elements can produce a low freezing point liquid at the center of the weld. This acts as a line of weakness and can cause cracking under the influence of transverse shrinkage strains. Impurity elements such as sulfur and phosphorus are essential in this type of cracking since they cause low melting point silicide and phosphides to be present in the weld metal. A schematic view of solidification cracking is shown in Figure 6.
Weld metals with low susceptibility to solidification cracking (low sulfur and phosphorous) are available for most structural steels, but cracking may still arise in the following circumstances:
- Suppose joint movement occurs during welding, e.g., due to distortion. A typical example of this is welding around a patch or nozzle. If the weld is continuous, the contraction of the first part of the weld imposes a strain while solidifying the rest.
- Suppose contamination of the weld metal with elements such as sulfur and phosphorus occurs. A typical example is welding articles with a Sulfur rich scale, such as a component in a sulfur-containing environment.
- Suppose the weld metal has to bridge a large gap, e.g., poor fit-up. In this case, the depth-to-width ratio of the weld bead may be small—contraction of the weld results in a significant strain being imposed on the center of the weld.
- If the parent steel is unsuitable, the diffusion of impurity elements from the steel into the weld metal can make it susceptible to cracking. Cracking susceptibility depends on the content of the alloying component with the parent metal and can be expressed in the following equation:
Note: The higher the number, the greater the susceptibility.
Solidification cracking can be controlled by careful choice of parent metal composition, process parameters, and joint design to avoid the circumstances previously outlined.
Heat Affected Zone (HAZ) Cracking
The parent material in the HAZ does not melt as a whole. Still, the temperature close to the fusion boundary may be so high that local melting can occur at grain boundaries due to constituents having a lower melting point than the surrounding matrix. This region may produce fine cracks if the residual stress is high. These cracks can be extended by fabrication stresses or during service. A schematic view of liquation cracking is shown in Figure 7.
The low melting point grain boundary films can be formed in steels from impurities such as sulfur, phosphorus, boron, arsenic, and tin. As with solidification cracking, increased carbon, sulfur, and phosphorous make the steel more prone to cracking.
There are two main ways of avoiding liquation cracking. First, care should be taken to ensure low Sulfur and phosphorus levels in the parent metal. Unfortunately, many steel specifications permit high enough levels of sulfur and phosphorus to introduce a risk of liquation cracking. Secondly, the welding process used affects the risk of liquation cracking.
Processes incorporating a relatively high heat input rate, such as submerged arc or electroslag welding, lead to a greater risk of liquation cracking than, for example, manual metal arc welding. This is the case since the HAZ spends longer at the liquation temperature (allowing greater segregation of low melting point elements), and a more significant amount of thermal strain accompanies welding.
Hydrogen Induced Cracking
This form of cracking (also known as HAZ, under bead, cold, or delayed cracking) occurs in the HAZ at temperatures less than 200°C. Cracks can form within minutes of welding or be delayed for several days. Three factors must co-exist if cracking is to occur. These factors are:
- The presence of hydrogen: Hydrogen is introduced into the molten weld pool during welding due to the decomposition of hydrogen-containing compounds in the arc, e.g., moisture, grease paint, and rust. Once the gas has dissolved in the weld metal, it can diffuse rapidly into the HAZ during cooling and ambient temperatures. In due course, the hydrogen will diffuse out of the steel. The diffusion can take weeks for a thick-walled vessel.
- A susceptible weld metal or HAZ: The cooling rate following most fusion welding processes is relatively rapid. This cooling can lead to the formation of martensite or other hardened structures in the HAZ and possibly the weld metal. Only small quantities of hydrogen can embrittle these structures.
- A high level of residual stress after welding: Cracking develops under the action of the residual stresses from welding in the susceptible microstructure of the HAZ or weld metal, where embrittlement has occurred due to the presence of hydrogen in the solution. A schematic view of hydrogen cracking in the HAZ of different weld designs is illustrated in Figure 8.
The methods of avoiding hydrogen cracking involve removing or limiting one of the three factors necessary for it to occur. Hydrogen cracking can be avoided by choosing a material that does not harden in the HAZ or weld metal with the particular welding process employed. The cooling rate controls the likelihood of hardening in the HAZ after welding and the hardenability of the parent steel.
The hardenability of steel is governed by its composition. A helpful way of describing hardenability is to assess the total contribution to it of all the elements present in the steel. This assessment is done by an empirical formula that defines a carbon equivalent value (CEV) and considers the essential factors that affect hardenability. A typical formula for the CEV (accepted by British Standards) is shown below:
As a general rule, hardening in the HAZ can be avoided by using steel with a CEV of less than 0,42, although it should be noted that the welding process parameters influence this value. Increasing the heat input rate of the welding process (where possible) is beneficial since it results in a slower cooling rate after welding and, therefore, a lower likelihood of hardening in the HAZ. For the same reason, there is less risk of hydrogen cracking when welding thin plates and sections since the cooling rate in the HAZ is less than in thick sections.
Limiting the presence of hydrogen by avoiding dampness, rust, and grease, using controlled hydrogen electrodes (properly dried basic coated electrodes), and low hydrogen welding processes (MIG or submerged arc welding) is another step towards avoiding cracking.
If these precautions are not sufficient, preheating is necessary. Preheating and maintaining a minimum interpass temperature during multi-pass welding has two effects. First, it results in softening of the HAZ because the cooling rate is reduced. Secondly, it accelerates hydrogen diffusion from the weld zone so that less remains after the weld has cooled. The minimum preheat temperature required to avoid hydrogen cracking depends on the chemical composition of the steel, the heat input rate, and the thicknesses being joined.
The minimum preheat temperature can be calculated by interrelating these facts in a welding procedure diagram. An example of one of these diagrams for carbon manganese steels is shown in Figure 9. This diagram is used in the following way :
- Select the appropriate heat input (arc energy) on the horizontal scale.
- Move vertically to intersect the appropriate combined thickness line for the joint design.
- Move horizontally from the intersection point to read off the preheat temperature for the CEV of the steel being.
Lamellar Tearing
This problem can arise if the residual stresses from welding are applied across the thickness of at least one of the plates being joined. Cracking occurs if the through-thickness elasticity of the plate is very low. A schematic view of this mode of cracking is shown in Figure 10.
Cracking typically occurs in the parent metal close to the outer boundary of the HAZ. The cracks have a characteristic stepped appearance, with the ‘threads’ of the steps parallel to the steel plate’s rolling direction. In contrast to hydrogen cracking, lamellar tears are not necessarily confined to the HAZ. In some cases, cracking can occur at the mid-thickness of a plate if a weld on both sides restrains it.
Lamellar tearing arises because the through-thickness ductility of the plate is reduced by the presence of planar inclusions lying parallel to the plate surface. All standard structural steels contain large inclusions that consist of non-metallic substances produced in the steelmaking process, e.g., sulphates and silicates.
These inclusions are formed as spheres, grain boundary films, or small angular particles in the steel ingot as it cools down after casting. When the ingot is rolled to make a steel plate, the inclusions deform into discs parallel to the plate surface. Different types of inclusions distort in different ways and break up during rolling. The form, distribution, and density of inclusions in a rolled plate determine the through-thickness ductility. Only a small proportion of steel plates have a sufficiently low through-thickness elasticity to be susceptible to lamellar tearing.
Lamellar tearing can be avoided in four main ways:
- Improved joint design: The fabrication design can be altered to prevent residual stresses in the through-thickness direction of a plate. Examples are shown in Figure 11.
- The use of forged products: The lamellar distribution of inclusions in a plate results from the plastic deformation occurring during rolling. The inclusion distribution in forged products is not so detrimental.
- Plate selection: The use of steel plates with a relatively low population of planar inclusions and thus adequate through-thickness ductility.
- Using a layer of low-strength weld metal: This reduces the strain transmitted through the thickness of the welded steel plates since the soft weld metal can deform plastically. This technique, known as ‘buttering,’ is relatively expensive but can be used when susceptible joints cannot be avoided.
Re-Heat Cracking
Removing or reducing residual stresses after welding by thermal stress relief is recommended for many fabrications. In this process, the joint reaches a temperature range where rapid creep can occur (about a third to a half of the melting point). As a result, the welding residual stresses are relieved by plastic deformation. Cracking can occur during this process if the weld or HAZ’s ductility is insufficient to accommodate the strain accompanying the residual stress relief. A schematic view of re-heat cracking is shown in Figure 12.
The residual tensile stress, which acts as the driving force for the cracking process, may be supplemented by transient thermal stresses in the weld zone. These stresses arise from rapid non-uniform heating up to the stress-relieving temperature. The presence of geometric stress raisers, e.g., toes of fillet welds, and pre-existing cracks, e.g., liquation and hydrogen cracks, accentuate the problem.
The cracking problem is prevalent during stress-relieving operations but can also occur in service situations. In such cases, the onset of cracking is expected to take much longer since the service temperature is generally significantly below the stress-relieving temperature.
Re-heat cracking is mainly confined to alloy steels containing substantial amounts of solid carbide-forming elements, e.g., Cr, Mo, and V. The presence of the alloy carbides inhibits grain boundary sliding and thus reduces high-temperature ductility. Cracking can usually be avoided by weld profiling, e.g., grinding away any geometric stress raisers, such as the toes of fillet welds, before heat treatment and by control of the heating rate to avoid high transient thermal stresses.
Structural steel can only be considered weldable if joints in the steel behave satisfactorily in service. To achieve adequate performance levels in structural applications, the integrity of the welded joint must be good. A high level of integrity can only be achieved if the welded joint microstructure possesses sufficient ductility to resist residual stresses, which arise from the welding thermal cycle, without cracking.
The chemical compositions of both the weld and parent metals (carbon equivalent value), together with the parameters of the welding process (heat input and cooling rates), are influential in determining joint ductility. The level of impurity elements, such as sulfur, phosphorous, and hydrogen, is particularly significant in deciding whether crack formation occurs during welding.