What defects are likely to occur when welding martensitic stainless steel pipes

Martensitic stainless steel pipes are widely used in the fields of oil, gas, chemical industry, aviation, shipbuilding and nuclear energy. They have high strength, good wear resistance and certain corrosion resistance, and are ideal for high-demand working conditions. Welding, as an important process link in connection and manufacturing, plays a vital role in the structural integrity and service life of martensitic stainless steel pipes. However, due to the unique metallographic structure and heat treatment characteristics of this material, a series of defects are easily generated during the welding process, affecting the performance and safety of use.

Cold cracks (quenching cracks)
Cold cracks are one of the most common and most dangerous defects when welding martensitic stainless steel pipes. This type of stainless steel contains high carbon and chromium, and martensitic transformation will occur during the cooling process of welding, resulting in large structural stress and residual stress. When the high-hardness martensitic structure is superimposed with tensile stress, delayed cracks or cold cracks are very likely to occur in the weld or heat-affected zone.
Cold cracks usually appear several hours or even days after welding, and are highly concealed and expand rapidly, seriously affecting the fatigue performance and safety of the structure. In order to avoid the occurrence of cold cracks, it is usually necessary to preheat the welding area and adopt appropriate tempering treatment.

Hot cracks (solid solution cracks)
Hot cracks mainly occur during the solidification process of the weld, which is caused by the shrinkage stress of the liquid metal exceeding the bonding strength of the grain boundary. Martensitic stainless steel contains a certain amount of impurity elements such as sulfur (S) and phosphorus (P), which form low-melting point eutectics at high welding temperatures and gather at the grain boundaries, reducing the grain boundary strength and increasing the risk of hot cracks.
Hot cracks are usually distributed linearly along the grain boundaries, with slender, deep and narrow shapes. They are not easy to detect in appearance and can only be found through X-ray or ultrasonic testing. Using low-sulfur and low-phosphorus welding materials, controlling heat input and optimizing welding parameters are important means to prevent hot cracks.

Hydrogen-induced cracks (delayed cracks)
If there is moisture, oil, rust or insufficiently dried welding materials during welding, hydrogen will be introduced. Hydrogen atoms dissolve in the weld metal at high temperatures, and gather in defects or inclusions during the cooling process to form high-pressure gas, which causes hydrogen-induced cracks.
Due to its high hardenability, martensitic stainless steel is highly sensitive to hydrogen and is very prone to hydrogen-induced cracking. This type of crack often occurs in the cooling stage after welding and can expand under static load or slight external load. The use of low-hydrogen welding process, preheating before welding, and slow cooling after welding are effective measures to reduce hydrogen-induced cracks.

Brittle failure caused by hardened structure
In the martensitic stainless steel welding area, especially the heat-affected zone (HAZ), due to local heating and rapid cooling, it is easy to form a high-hardness brittle martensitic structure, even accompanied by carbide precipitation, resulting in a sharp decrease in local toughness.
If the high-hardness area is not properly tempered, it is very easy to cause brittle fracture under impact load or fatigue load. The embrittlement of the heat-affected zone is usually one of the root causes of welding failure and is also a key control item in welding process assessment.

Oxidation inclusions and incomplete fusion defects
If sufficient shielding gas or improper shielding method is not used during the welding of martensitic stainless steel, the weld metal will be severely oxidized, forming oxide inclusions and reducing the purity of the weld metal. Oxidation inclusions not only reduce strength, but also become crack sources, which are easy to induce failure during service.
At the same time, too low welding heat input, poor groove preparation or poor operation technology may all lead to incomplete fusion or incomplete penetration defects. Such defects reduce the load-bearing cross-sectional area of the structure and are important factors in causing fatigue cracks and early fractures.

Excessive deformation and residual stress
Due to the phase change expansion and contraction during the welding process of martensitic stainless steel, the stress field is complex, and large residual stress and welding deformation are easily formed after welding. If not controlled, it will not only affect the dimensional accuracy of the pipeline or structure, but may also cause stress corrosion cracking.
By controlling heat input, adopting a reasonable welding sequence, appropriate fixture positioning and post-weld heat treatment, deformation can be effectively reduced and residual stress can be released.

Welding porosity and pores
If there is moisture, oil or unstable shielding gas during welding, porosity defects will occur. Most of these pores are distributed inside the weld. Although they are small in size, they can easily become stress concentration points in high pressure or corrosive environments.
Pores may also affect the density and sealing of welds, especially in pipelines that transport gas or high-pressure liquids. Their presence will seriously affect the safe operation of the system.