1.1 Effect of chromium on the performance of stainless steel and its welds

Chromium is added primarily to provide corrosion resistance to the steel. This is particularly effective for oxidizing environments such as those containing nitric acid. After adding chromium, an oxide with a chemical ratio of (Fe, Cr) 2O3 is formed on the surface of the steel. Because chromium and oxygen have much higher affinity than iron, the presence of chromium increases the stability of this oxide. Steels with a mass fraction of chromium above 11% are considered stainless in the atmospheric environment. Stable oxides in more aggressive environments require higher chromium levels.

Chromium is a strong ferrite-forming element and is the only element industrially available to make stainless steel stainless. Chromium shrinks the austenite phase region. When its content is greater than about 12%, the austenite phase region completely disappears. This means that alloys containing 12% or more of Cr do not undergo γ → α transformation, and therefore do not cause grain refinement and hardening. Therefore, FeCr-based alloys with w (Cr) greater than 12% are all ferritic alloys. Increasing the chromium content in FeCrC or FeCrNiC alloys will promote the formation and retention of ferrite in martensitic stainless steel, austenitic stainless steel and duplex stainless steel. In ferritic alloy steels, chromium is the main alloying element that stabilizes the ferrite structure. Chromium is also an important component in the formation of intermetallic compounds, which tend to embrittle stainless steel.

In short, the atomic radii of chromium and iron are very close, and their electronegativity is almost the same, so the two can form a continuous solid solution. The addition of chromium to iron can cause major changes in the critical point and the structure of the iron (body-centered cubic and face-centered cubic). At the same time, chromium can be used as a substitution atom in the body-centered cubic (BCC) and face-centered cubic (FCC) lattices, so from a mechanical point of view, chromium can play a solid solution strengthening role to a certain extent. However, in ferritic alloy steels, especially when carbon and nitrogen are present in the steel, high chromium content makes the toughness and plasticity very poor. High chromium ferritic steels must be carefully treated or the carbon and nitrogen content must be reduced to obtain acceptable mechanical properties after welding.

1.2 Effect of Molybdenum on the performance of stainless steel and its weld

Molybdenum is added to many stainless steels, and the role of molybdenum varies depending on the type of steel. Molybdenum can be added to 6% in ferritic, austenitic, and duplex stainless steels. For super stainless steels, the amount can be higher. This is to improve corrosion resistance, especially to improve pitting and crevice corrosion resistance. Molybdenum can also improve high-temperature strength in austenitic stainless steel. For example, adding 2% molybdenum to 18Cr8Ni steel can increase the high-temperature strength at 760 ° C by 40%. However, molybdenum also has side effects, because hot processing of molybdenum-containing steels is more difficult. In some martensitic stainless steels, molybdenum is added to form carbides. Only adding 0.5% of molybdenum can improve the secondary hardening performance of steel, increase the yield strength and tensile strength at room temperature, and improve the high temperature performance.

Like chromium, molybdenum shrinks the γ phase region, which means that molybdenum promotes ferrite formation and is a ferrite-forming element. 11.5% of chromium can completely disappear the γ-phase region. For molybdenum, only 2.9% can make the γ-phase region disappear. Molybdenum and iron can form an intermetallic phase, the most important of which is the Laves phase (Fe2Mo) containing about 45% of molybdenum. When the amount of molybdenum reaches 5%, this phase will precipitate. The intermetallic compound χ phase (Fe36Cr12Mo10) can also be generated in iron-chromium-molybdenum alloys. Due to the presence of chromium, the χ phase moves to a low molybdenum content. When the chromium content is 17%, the χ phase starts to precipitate from 3% molybdenum. When high, the Laves phase will also precipitate. In the iron-chromium series, the temperature of the χ phase precipitation zone is higher than that of the σ phase precipitation zone, which can explain why stainless steel containing molybdenum requires a higher temperature solution annealing treatment than stainless steel without molybdenum. The toughness and corrosion resistance of these metals relative to molybdenum-containing stainless steels and weld metals are detrimental.

1.3 Effect of silicon on the performance of stainless steel and its weld

Silicon is actually contained in all stainless steels. Silicon is mainly added to deoxidize metals during smelting. In most stainless steels, its mass fraction is 0.3% to 0.6%. Aluminum is sometimes used as a deoxidizer instead of silicon, but it is rare in stainless steel. It has been found that the addition of silicon in a mass fraction of 4% to 5% can improve the corrosion resistance, and the addition of silicon in a mass fraction of 1% to 3% in some heat-resistant steels can improve the ability to prevent the formation of scale at high temperatures. The role of silicon in promoting ferrite or austenite formation is not fully understood. The content of silicon in the austenitic stainless steel with a mass fraction of 1% or less does not seem to have an effect on the phase equilibrium, but higher content will promote the formation of ferrite. Silicon also promotes ferrite formation in ferritic and martensitic stainless steels. Therefore, silicon is generally regarded as a ferrite forming element. In the phase-changeable martensitic stainless steel, the addition of silicon will promote the formation of ferrite. At this time, the composition of the steel must be controlled to avoid the formation of a single ferrite structure. Loss of hardenability. In austenitic steel, as the amount of silicon increases, the amount of delta ferrite will increase, and the formation of intermetallic phases (σ or χ) will also accelerate and increase, which may cause embrittlement. In order to ensure a pure austenite structure, while increasing the amount of silicon, the austenite-forming elements (Ni, N, etc.) in the steel and in the weld should be increased accordingly. Π-phase [M11 (CN) 2] and Cr3Si (β-phase) similar to M23C6 may exist in austenitic stainless steels containing silicon. The π-phase precipitates in the steel, which slows down the precipitation of the σ-phase. Therefore, the addition of nitrogen has a positive effect on the austenitic nickel chromium stainless steel containing silicon. Silicon tends to form a low-melting phase, so it has a greater effect on the thermal cracking of the weld. In steels that crystallize into the γ phase at one time, due to the segregation phenomenon, low-melting-point phases are mainly formed at the grain boundaries of the austenite grains that are precipitated at one time. In the weld metal, when the δ ferrite phase is formed by one solidification, the harmful effect of silicon is greater than when the γ phase is formed by one solidification. Silicon is an alloying element in stainless steel. When its content is 4% to 5%, it can greatly improve the strong nitric acid corrosion resistance of CrNi austenitic steel. When it is added in an amount of 1% to 3%, its oxidation resistance can be improved.

In addition, because silicon can improve the fluidity of molten steel, the amount of silicon added to the filler for welding can be slightly higher than the conventional value. Some stainless steels, especially austenitic stainless steels, are very viscous in the molten state, so adding silicon can greatly improve their fluidity.

1.4 Effect of Aluminium on the Performance of Stainless Steel and Welds

Aluminum is a ferrite-forming element, and its ability to form ferrite is 2.5 to 3 times that of chromium. In most stainless steels and their welds, aluminum is added as a deoxidizer. Because aluminum-containing inclusions increase the sensitivity of the steel to pitting corrosion, aluminum is generally not used as a deoxidizer. As alloying elements, they are used for different purposes in different steels, but the main role of aluminum is to age strengthen and improve the tempering stability and increase the secondary hardening effect.