Why Do Stainless Steel Flanges Crack During Welding?

A flange is a commonly used pipe component. When flanges and pipes are connected, welding is often used. However, if improper operation is performed during welding, cracks will occur between the flange and the stainless steel pipe. Why does the flange crack during welding? How to avoid flange cracking? Let’s take a look at it together. I hope that these contents can help everyone better understand the things that need to be paid attention to during the welding of pipe fittings such as flanges.
 
1. Types of cracks of flanges
Before knowing the cause of welding cracks in flanges, you first need to know the type of crack, hot cracks or cold cracks. Depending on the type of crack, the causes vary. Flange welding cracks occur when the flange and stainless steel pipe are welded. Welding is a manufacturing process and technology that uses heat, high temperature, or high pressure to connect metals and other thermoplastic materials such as plastics, so flange welding cracks resemble heat cracks. Here, we mainly analyze the causes of hot cracks of flanges, because welding generally causes hot cracks.
 
Hot cracks are cracks produced during the high-temperature stage of the welding cooling process and mainly exist in the welded metal. A small amount also exists near the welding seam, which is divided into crystallization (solidification) cracks, liquefaction cracks and polygonal cracks. Among them, crystal cracks are common cracks, which mainly occur in the welding of carbon steel with many impurity elements.
 
2. Causes of welding cracks of flanges 
There are many reasons for flange welding cracks. It can be caused by the flange or the stainless steel pipe itself. It can also be caused by the tensile stress of welding and the degree of overheating of the welded joint. 

(1) Effect of carbon content on flange cracking
When the carbon content exceeds 0.04%, carbides will be produced. The more carbides are, the lower the corrosion resistance of the steel and the greater the brittleness become. If the flange is welded with stainless steel pipes at this time, cracks will occur.
 
(2) The influence of thermal cracks on flange cracking
Hot cracks are cracks that occur at high temperatures in the welding process. They mainly appear in the metal of the welding seam and can be divided into crystallization cracks, liquefaction cracks and polygonal cracks. Crystallization cracks are the most common type and mainly occur in single-phase austenitic stainless steel, carbon steel containing more impurity elements such as S, P, Si and C, and in the welding seam of other welded structures such as aluminum and its alloys. The main influencing factors are the degree of overheating of the welded joint, low melting point eutectic (chemical composition of the welding seam metal), and welding tensile stress.
 
(3) The influence of reheat cracks on flange cracking
It usually occurs in certain steel grades containing precipitation-strengthening elements and high-temperature alloys, including low-alloy high-strength steels, pearlitic heat-resistant steels, precipitation-strengthened high-temperature alloys, and some austenitic stainless steels. No cracks were found after they were welded, but cracks developed during the heat treatment. To prevent reheat cracking, fine-grained steels can be used.

(4) The influence of cold cracks on flange cracking
It mainly occurs in the welding heat-affected zone of high and medium carbon steel, low and medium alloy steel, but in some metals, such as some ultra-high-strength steels, titanium and titanium alloys, sometimes cold cracks also occur in welding seams. Usually, the hardening tendency of the steel type, the hydrogen content and distribution of the welded joint, and the constrained state of the welded joint are the three main factors that cause cold cracks in the welding of high-strength steel. The martensite structure formed after welding combines with tensile stress under the action of hydrogen to form cold cracks. Its formation is usually through the grain or along the grain. Cold cracks are generally divided into cracks under the welding bead, welding toe cracks, and root cracks.
 
(5) Effect of welding stress on flange cracking
Welding consists of two parts: heating and cooling. When the heat source moves along the welding object, the temperature at a certain point on the welding object goes from low to high over time, and then from high to low when it reaches the limit value, forming a welding thermal cycle. During welding, due to the concentrated intensity of the heat source, the heating speed is much faster than the heat treatment conditions, and the partially heated area continues to move. Therefore, the welding area close to the flange is subject to complex welding stress during welding, causing cracks.
 
3. Solutions to flange cracking
Preventing and controlling flange cold cracks can start from these aspects: the selection of welding materials and process measures. Welding rods containing low hydrogen should be used, and the welding seam should be matched with low strength. For materials with great cold cracking tendency, austenitic welding materials can also be used; materials with lower carbon content should be used as much as possible. These are measures to prevent cold cracking.
 
Reduce the content of impurity elements such as S and P in the material and appropriately increase the Mn/S ratio. Replace Fe in the Fe-FeS low melting point eutectic to form MnS with a melting point of 1620Co, thus improving the crack resistance of the welding seam. Use appropriate welding methods and processes to control line energy input and reduce overheating of the welding seam. Add Ti, Mo, Nb or rare earth elements to the welding materials to inhibit the development of columnar grains, refine the grains, and significantly improve performance.
 
The above is some knowledge about the cracking of stainless steel flange pipe fittings. We hope that the above knowledge can help avoid cracks and improve the quality of the product.