The lamellar tearing is a kind of internal low-temperature cracking. It is limited to the base metal of thick plates or the heat-affected zone of welding seams, which mostly occurs in "L", "T", and "+" shaped joints. It is defined as a kind of stepped cold crack that occurs in the base metal when the rolled thick steel plate doesn't have enough plasticity to withstand the welding shrinkage strain along the thickness direction. Generally, the reason is that in the rolling process of the thick steel plate, some non-metallic inclusions in the steel are rolled into band-shaped inclusions parallel to the rolling direction. These inclusions cause the different mechanical properties of the steel plate in various directions.
To prevent lamellar tearing, refined steel can be used for valves, that is, steel plates with high z-direction performance can be selected, and the joint design can also be improved to avoid single-sided welding seams or beveled on the side bearing z-direction stress. Lamellar tearing is different from cold cracking. Its occurrence has nothing to do with the strength of the steel grade, but is mainly related to the amount and distribution of inclusions in the steel. Generally, lamellar tearing occurs in rolled thick steel plates, such as low-carbon steel plates, low-alloy high-impact steel plates, and even aluminum alloy plates. According to the location of lamellar tearing, lamellar tearing can be divided into three categories: lamellar tearing induced by cold cracks in the weld toe or weld root in the heat-affected zone of welding, cracking along the inclusions in welding heat-affected zone (the most common lamellar tearing in engineering), and cracking along the inclusions in the base metal away from the heat-affected zone (generally in thick plates with more MnS flake inclusions). The form of lamellar tearing is closely related to the type, shape, distribution, and location of inclusions. When the sheet-like MnS inclusions are dominant along the rolling direction, the laminar tear has a clear step shape, and when the silicate inclusions are dominant, it is linear. For example, when the Al inclusions are dominant, it shows irregular steps. When welding is performed for thick plates, especially T-shaped and corner joints, the shrinkage of the welding seam will produce great tensile stress and strain in the thickness direction of the base metal under the conditions of rigid constraints. When the strain exceeds the plastic deformation of the base metal, the inclusions and metal matrix will be separated and microcracks will occur. Under the continued action of stress, the crack tips will expand along the plane where the inclusions are located.
There are many factors that affect lamellar tearing, which are mainly the following aspects:
- The type, quantity and distribution of non-metallic inclusions are the essential cause of lamellar tearing, which is the root cause of the anisotropy and mechanical properties of steel.
- Thick-walled welded structures bear different Z-direction restraint stresses, residual stresses and loads in the welding process, which are the mechanical conditions that cause lamellar tearing.
- It is generally believed that in the vicinity of the heat-affected zone, lamellar tearing is induced by cold cracking, and hydrogen is an important influencing factor.
It is necessary to judge the sensitivity of the steel's lamellar tearing before construction due to the great impact of lamellar tearing and great harm. Commonly used evaluation methods are Z-direction tensile reduction of area and bolt Z-direction critical stress. In order to prevent lamellar tearing, the reduction of the area should not be less than 15%; generally, between 15 and 20% is appropriate, when it is 25%, it is considered that the lamellar tearing resistance is excellent.
To prevent lamellar tearing, measures should be taken mainly from the following aspects:
First, the method of pre-desulfurization of molten iron and vacuum degassing should be widely used, which can smelt ultra-low sulfur steel with a sulfur content of only 0.003 to 0.005%. Its reduction rate (Z-direction) can reach 23 to 25%.
Second, control the form of sulfide inclusions to change MnS into sulfides with other elements, making it difficult to stretch during hot rolling, thereby reducing anisotropy. The currently widely used additive elements are calcium and rare earth elements. After the steel is treated by the above-mentioned treatment, a lamellar tear-resistant steel plate with a Z-direction reduction of area of 50 to 70% can be manufactured.
Third, from the perspective of preventing lamellar tearing, when it comes to the design and construction process, avoid Z-direction stress and stress concentration. Specific measures are as follows:
- Single-sided welding seams should be avoided as much as possible, and double-sided welding seams should be used instead to ease the stress state of the root area of the welding seam and to prevent stress concentration.
- Use symmetric fillet welding seams with less welding instead of full welding seams with much welding to avoid excessive stress.
- Grooving should be made on the side bearing the Z-direction stress.
- For T-shaped joints, a layer of low-strength welding material can be built up welded in advance on the horizontal plate to prevent root cracks and at the same time alleviate welding strain.
- To prevent lamellar tearing caused by cold cracking, some measures to prevent cold cracking should be adopted as far as possible, such as reducing the amount of hydrogen, appropriately increasing the preheating, and controlling the temperature between layers.
