In metalworking, a welding defect is any flaw that compromises the usefulness of a weldment. There are many different types of welding defects, which are classified according to ISO 6520, while acceptable limits for welds are specified in ISO 5817 and ISO 10042.
Major causes According to the American Society of Mechanical Engineers (ASME), the causes of welding defects can be classified as follows: 41% poor process conditions, 32% operator error, 12% using the wrong technique, 10% incorrect consumables, and 5% bad weld grooves.
Hydrogen embrittlement
Residual stresses
The magnitude of residual stress caused by the heating, and subsequent cooling, from welding can be roughly calculated using:
E α Δ T {\displaystyle E\alpha \Delta T}
Where E {\displaystyle E} is Young's modulus, α {\displaystyle \alpha } is the coefficient of thermal expansion, and Δ T {\displaystyle \Delta T} is the temperature change. This approximates 3.5 GPa (510,000 psi) for steel.
Types
Cracks
Arc strikes An arc strike is a discontinuity resulting from an arc consisting of any localized remelted metal, heat-affected metal, or change in the surface profile of any metal object. Arc strikes result in localized base metal heating and very rapid cooling. When located outside the intended weld area, they may result in hardening or localized cracking and may serve as potential sites subsequent fracturing. In statically loaded structures, arc strikes need not be removed unless such removal is required in contract documents. However, in cyclically loaded structures, arc strikes may result in stress concentrations that would be detrimental to the serviceability of such structures, and arc strikes should be ground smooth and visually inspected for cracks.
Cold cracking Cold cracking—also known as delayed cracking, hydrogen-assisted cracking (HAC), or hydrogen-induced cracking (HIC)—is a type of defect that often develops after solidification of the weld when the temperature starts to drop from about 190 °C (375 °F); the phenomenon often arises at room temperature, and it can take up to 24 hours to appear even after complete cooling. Some codes require testing of welded objects 48 hours after the welding process. This type of crack is usually observed in the heat affected zone (HAZ), especially with carbon steel, which has limited hardenability. For other alloy steels, with a high degree of hardenability, cold cracking could occur in both the weld metal and the HAZ. This crack mechanism can also propagate between grains and through grains. Factors that can contribute to the occurrence of cold cracking are:
The amount of hydrogen (H2) dissolved in weld metal: Dissolved hydrogen in the weld metal is related to hydrogen embrittlement. Hydrogen content can be reduced by using hydrogen-free consumables. In the case of welding filler (especially in shielded metal arc welding (SMAW)) exposed to the atmosphere, proper electrode baking is recommended to eliminate moisture from flux. Preheating of the base material is also one of the techniques used to release hydrogen from the working object. Residual tensile stress: Residual tensile stress can cause cracks to propagate without any applied stress. This can be avoided by preheating the base metal, which reduces the different thermal expansion coefficients that will affect the cooling rate of weld metal. Utilizing low-yield-strength filler metal is also preferable because the magnitude of residual stresses can be equal to the σ yield of the metal. Therefore, the use of austenitic stainless steel or nickel-base filler could be considered due to its ductile nature. Also, post weld heat treatment (PWHT) will release any residual stresses on the weld joint. Hardness of weld metal and heat affected zone (HAZ): Hardness is correlated with the brittleness of the material. To reduce excessive hardness, preheating and PWHT can be applied to the object. Hardness values below 350 VHN have less tendency to crack. Structure of weld metal and HAZ: Cold cracking in steels is associated with forming martensite as the weld cools. Hydrogen has very low solubility in martensite, which can lead to the gas being trapped inside the weld if care isn't taken. Slower cooling rates during the welding process help to avoid martensite formation. In addition, a slower cooling rate means a longer time at an elevated temperature, which allows more hydrogen to escape. A slower cooling rate is achieved by using high heat input and maintaining it during the welding. The alloy composition of the base metal also has an essential role in the likelihood of a cold crack occurring, since that composition relates to the hardenability of materials. With high cooling rates, the risk of forming a hard, brittle structure in the weld metal and HAZ is more likely. 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.
CE IIW = C + Mn / 6 + ( Cr + Mo + V ) / 5 + ( Ni + Cu ) / 15 {\displaystyle {\ce {CE_{IIW}= C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15}}} (concentration is shown as percentage of weight) Then, depending on the carbon content (with additional elements influencing the carbon equivalent index), steels can be classified into three zones, from their cold cracking behavior, as shown in the Graville diagram.
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