阴APP导航

In a previous article, Part 1,听 on the weldability of Stainless Steels, we dealt with the weldability of the Austenitic and Ferritic types. This article, Part 2, will deal with the weldability of:听

• Martensitic Stainless Steels

• Duplex Stainless Steels

• Precipitation Hardening Stainless Steels

As previously stated, it鈥檚 beyond the scope of these articles to delve deeply into complex metallurgy. The objective is to point out the areas that one needs to be aware of, to highlight the possible pitfalls that need to be overcome when welding and, to illustrate that all stainless steels are not made the same

Welding Martensitic Stainless Steels

Martensitic stainless steels can be welded with matching electrodes that produce a martensitic weld deposit, or with austenitic electrodes.

• If the weld metal must match the base material in hardness and wear resistance then martensitic weld metal must be used.听

• If, however, hardness is not a consideration and only corrosion resistance is a factor, then the use of austenitic electrodes is a valid choice.

The chief difficulty in welding martensitic steels is the avoidance of HICC, (hydrogen induced cold cracking). Cracking is the result of the high hardness of martensite in the weld zone and is assisted by the presence of hydrogen. The welding procedure must control sources of hydrogen such as moisture, dirt, grease and other contaminants, and an adequate preheat must be applied and maintained. These steels are very hardenable and the preheats main objective is to assist in removing hydrogen from the weld zone.

In most cases, when welding with matching electrodes, a post weld heat treatment is required and this would form part of the welding procedure.

Austenitic electrodes of the E309/309L austenitic type may be specified when joining a martensitic stainless steel to another material such as carbon steel or austenitic stainless steel. A typical procedure would involve buttering the martensitic steel with 309 to form a layer of austenitic weld metal before welding to the other material. A preheat may still be necessary to avoid cracking in the heat affected zone of the martensitic steel.

Welding Duplex Stainless Steels

As the name suggests, duplex stainless steels rely on a balance of two metallurgical phase鈥檚, austenite and ferrite in the metals matrix which provide a unique combination of properties. The optimum phase balance is normally considered to be about 50:50.听

This 50:50 balance is relatively easy to produce at the mill but is less so when welding, especially if uncontrolled. Numerous problems can arise when the austenite to ferrite ratio is not correct. For example:听

• Relatively little austenite can lead to poor toughness and loss of general corrosion resistance听

• Excessive austenite will lower the strength and can impair stress corrosion cracking resistance.听

Figure 1 illustrates pitting attack on a tube-to-tube sheet weld where the metallurgical phase balance was incorrect due to the use of a faulty welding procedure.

Generally speaking, welding is carried out with the highest possible heat input that is consistent with maintaining properties and weldability. This lowers the cooling rate at the higher temperatures, which allows more time for ferrite to transform to austenite during cooling and will improve the final phase balance. The weldment should be allowed to cool between passes to limit the time in a temperature range that promotes the formation of a brittle phase.

Again, all these items will be part of a welding procedure which should be rigorously followed by the practitioner.

听

Figure 1 Pitting Attack on a Tube-to-Tube Sheet Welds. Typical Ares shown by Red Arrows

Matching filler metals are often used for duplex stainless steels, but with somewhat higher nickel content. Nitrogen may also be added via the filler metal or in the shielding gas. Nickel and nitrogen help to help promote transformation to austenite upon cooling and help in providing an optimum austenite content in the solidified weld metal.

Welding Precipitation Hardening Stainless Steels

The precipitation hardening stainless steels rely for their strength on a heat treatment that results in precipitation of small particles in the general metallurgical matrix. This heat treatment is known as aging and is metallurgically complex. This fact controls every approach to welding of these PH stainless steels as the heat from welding will upset the effects of the initial heat treatment performed at the mill.听

The damage, a softening, occurs in two areas during welding:

• Close to the fusion boundary is an area in which precipitates are re-dissolved (solution treated), and听

• Further from the fusion boundary there will exist a region that becomes overaged as shown听

These areas are depicted in Figure 2.听

听

Figure 2. Areas in the Weld Zone that are Affected听 by the Heat of Welding

The consequence of this is that to achieve the various strength levels required by design, a post weld heat treatment is usually mandatory.听

• The martensitic and semi-austenitic PH stainless steel types are relatively easy to weld, and听 preheat is not required. Post weld heat treatment will depend on design requirements and will be reflected in the welding procedure

• The austenitic PH types are difficult to weld because of the possibility of hot cracking in the heat affected zone. They are often welded using nickel-based filler metals. Again, post weld heat treatment will depend on design requirements

Inert gas shielded processes, GTAW and GMAW in argon with 2% oxygen are most commonly employed. As stated, these alloys have a complex metallurgy and welding must follow a prescribed and approved welding procedure

As mentioned in the opening paragraph, the objective of these two articles (Part 1 and 2) on the welding of stainless steels was simply to give some background into these alloys and to point out that all stainless steels are not the same. The different alloys systems demand differing approaches to their welding and, as such, need to be respected during fabrication.