The stainless properties of stainless steels are primarily due to the presence of chromium in quantities greater than roughly 12 weight percent. This level of chromium is the minimum level of chromium to ensure a continuous stable layer of protective chromium-rich oxide forms on the surface. The ability to form chromium oxide in the weld region must be maintained to ensure stainless properties of the weld region after welding. In commercial practice, however, some stainless steels are sold containing as little as 9 weight percent chromium and will rust at ambient temperatures.
Stainless steels are generally classified by their microstructure and are identified as ferritic, martensitic, austenitic, or duplex (austenitic and ferritic). The microstructure significantly affects the weld properties and the choice of welding procedure used for these stainless steel alloys. In addition, a number of precipitation-hardenable (PH) stainless steels exist. Precipitation-hardenable stainless steels have martensitic or austenitic microstructures.
Iron, carbon, chromium and nickel are the primary elements found in stainless steels and significantly affect microstructure and welding. Other alloying elements are added to control microstructure or enhance material properties. These other alloys affect welding properties by changing the chromium or nickel equivalents and thereby changing the microstructure of the weld metal. Generally, 200 and 300 series alloys are mostly austenitic and 400 series alloys are ferritic or martensitic, but exceptions exist.
Stainless steels are subject to several forms of localized corrosive attack. The prevention of localized corrosive attack is one of the concerns when selecting base metal, filler metal and welding procedures when fabricating components from stainless steels.
Stainless steels are subject to weld metal and heat affected zone cracking, the formation of embrittling second phases and concerns about ductile to brittle fracture transition. The prevention of cracking or the formation of embrittling microstructures is another main concern when welding or fabricating stainless steels.


Welding Austenitic Stainless Steels
Ideally, austenitic stainless steels exhibit a single-phase, the face-centered cubic (fcc) structure, that is maintained over a wide range of temperatures. This structure results from a balance of alloying additions, primarily nickel, that stabilize the austenite phase from elevated to cryogenic temperatures. Because these alloys are predominantly single phase, they can only be strengthened by solid-solution alloying or by work hardening. Precipitation-strengthened austenitic stainless steels will be discussed separately below.
The austenitic stainless steels were developed for use in both mild and severe corrosive conditions. Austenitic stainless steels are used at temperatures that range from cryogenic temperatures, where they exhibit high toughness, to elevated temperatures, where they exhibit good oxidation resistance. Because the austenitic materials are nonmagnetic, they are sometimes used in applications where magnetic materials are not acceptable.
The most common types of austenitic stainless steels are the 200 and 300 series. Within these two grades, the alloying additions vary significantly. Furthermore, alloying additions and specific alloy composition can have a major effect on weldability and the as-welded microstructure. The 300 series of alloys typically contain from 8 to 20 weight percent Ni and from 16 to 25 weight percent Cr.
A concern, when welding the austenitic stainless steels, is the susceptibility to solidification and liquation cracking. Cracks can occur in various regions of the weld with different orientations, such as centerline cracks, transverse cracks, and microcracks in the underlying weld metal or adjacent heat-affected zone (HAZ). These cracks are primarily due, to low-melting liquid phases, which allow boundaries to separate under the thermal and shrinkage stresses during weld solidification and cooling.
Even with these cracking concerns, the austenitic stainless steels are generally considered the most weldable of the stainless steels. Because of their physical properties, the welding behavior of austenitic stainless steels is different than the ferritic, martensitic, and duplex stainless steels. For example, the thermal conductivity of austenitic alloys is roughly half that of ferritic alloys. Therefore, the weld heat input that is required to achieve the same penetration is reduced. In contrast, the coefficient of thermal expansion of austenite is 30 to 40 percent greater than that of ferrite, which can result in increases in both distortion and residual stresses, due to welding. The molten weld pool of the austenitic stainless steels is commonly more viscous, or sluggish, than ferritic and martensitic alloys. This slows down the metal flow and wettability of welds in austenitic alloys, which may promote lack-of-fusion defects when poor welding procedures are employed.

Welding Ferritic Stainless Steels
Ferritic stainless steels comprise approximately half of the 400 series stainless steels. These steels contain from 10.5 to 30 weight percent chromium along with other alloying elements, particularly molybdenum. Ferritic stainless steels are noted for their stress-corrosion cracking (SCC) resistance and good resistance to pitting and crevice corrosion in chloride environments, but have poor toughness, especially in the welded condition.
Ideally, ferritic stainless steels have the body-centered cubic (bcc) crystal structure known as ferrite at all temperatures below their melting temperatures. Many of these alloys are subject to the precipitation of undesirable intermetallic phases when exposed to certain temperature ranges. The higher-chromium alloys can be embrittled by precipitation of the tetragonal sigma phase, which is based on the compound FeCr.
Molybdenum promotes formation of the complex cubic chi phase, which has a nominal composition of Fe36Cr12Mo10. Embrittlement increases with increasing chromium plus molybdenum contents. It is generally agreed that the severe embrittlement which occurs upon long-term exposure is due to the decomposition of the iron-chromium ferrite phase into a mixture of iron-rich alpha and chromium-rich alpha-prime phases. This embrittlement is often called "alpha-prime embrittlement." Additional reactions such as chromium carbide and nitride precipitation may play a significant role in the more rapid, early stage 885 °F embrittlement.
The ferritic stainless steels have higher yield strengths and lower ductilities than austenitic stainless steels. Like carbon steels, and unlike austenitic stainless steels, the ferritic stainless alloys exhibit a transition from ductile-to-brittle behavior as the temperature is reduced, especially in notched impact tests. The ductile-to-brittle transition temperature (DBTT) for the ultrahigh-purity ferritic stainless steels is lower than that for standard ferritic stainless steels. It is typically below room temperature for the ultrahigh-purity ferritic stainless steels. Nickel additions lower the DBTT and there by slightly increase the thicknesses associated with high toughness. Nevertheless, with or without nickel, the ferritic stainless steels would need engineering review for anything other than thin walled applications as they are prone to brittle failure.

Welding Martensitic Stainless Steels
Martensitic stainless steels are considered to be the most difficult of the stainless steel alloys to weld. Higher carbon contents will produce greater hardness and, therefore, an increased susceptibility to cracking.
In addition to the problems that result from localized stresses associated with the volume change upon martensitic transformation, the risk of cracking will increase when hydrogen from various sources is present in the weld metal. A complete and appropriate welding process is needed to prevent cracking and produce a sound weld.
Martensitic stainless steels are essentially alloys of chromium and carbon that possess a body-centered cubic (bcc) or body-centered tetragonal (bct) crystal structure (martensitic) in the hardened condition. They are ferromagnetic and hardenable by heat treatments. Their general resistance to corrosion is adequate for some corrosive environments, but not as good as other stainless steels.
The chromium content of these materials generally ranges from 11.5 to 18 weight percent, and their carbon content can be as high as 1.2 weight percent. The chromium and carbon contents are balanced to ensure a martensitic structure after hardening. Martensitic stainless steels are chosen for their good tensile strength, creep, and fatigue strength properties, in combination with moderate corrosion resistance and heat resistance.
The most commonly used alloy within this stainless steel family is type 410, which contains about 12 weight percent chromium and 0.1 weight percent carbon to provide strength. Molybdenum can be added to improve mechanical properties or corrosion resistance. Nickel can be added for the same reasons. When higher chromium levels are used to improve corrosion resistance, nickel also serves to maintain the desired microstructure and to prevent excessive free ferrite. The limitations on the alloy content required to maintain the desired fully martensitic structure restrict the obtainable corrosion resistance to moderate levels.

Welding Duplex Stainless Steels
Duplex stainless steels are two phase alloys based on the iron-chromium-nickel system. Duplex stainless steels usually comprise approximately equal proportions of the body-centered cubic (bcc) ferrite and face-centered cubic (fcc) austenite phases in their microstructure and generally have a low carbon content as well as, additions of molybdenum, nitrogen, tungsten, and copper. Typical chromium contents are 20 to 30 weight percent and nickel contents are 5 to 10 weight percent. The specific advantages offered by duplex stainless steels over conventional 300 series stainless steels are strength, chloride stress-corrosion cracking resistance, and pitting corrosion resistance.
Duplex stainless steels are used in the intermediate temperature ranges from ambient to several hundred degrees Fahrenheit (depending on environment), where resistance to acids and aqueous chlorides is required. The weldability and welding characteristics of duplex stainless steels are better than those of ferritic stainless steels, but generally not as good as austenitic materials.
A suitable welding process is needed to obtain sound welds. Duplex stainless steel weldability is generally good, although it is not as forgiving as austenitic stainless steels. Control of heat input is important. Solidification cracking and hydrogen cracking are concerns when welding duplex stainless steels, but not as significant for some other stainless steel alloys.
Current commercial grades of duplex stainless steels contain between 22 and 26 weight percent chromium, 4 to 7 weight percent nickel, up to 4.5 weight percent molybdenum, as well as some copper, tungsten, and nitrogen. Modifications to the alloy compositions have been made to improve corrosion resistance, workability, and weldability. In particular, nitrogen additions have been effective in improving pitting corrosion resistance and weldability.
The properties of duplex stainless steels can be appreciably affected by welding. Due to the importance of maintaining a balanced microstructure and avoiding the formation of undesirable metallurgical phases, the welding procedures must be properly specified and controlled. If the welding procedure is improper and disrupts the appropriate microstructure, loss of material properties can occur.
Because these steels derive properties from both austenitic and ferritic portions of the structure, many of the single-phase base material characteristics are also evident in duplex materials. Austenitic stainless steels have good weldability and low-temperature toughness, whereas their chloride SCC resistance and strength are comparatively poor. Ferritic stainless steels have good resistance to chloride SCC but have poor toughness, especially in the welded condition. A duplex microstructure with high ferrite content can therefore have poor low-temperature notch toughness, whereas a structure with high austenite content can possess low strength and reduced resistance to chloride SCC.
The high alloy content of duplex stainless steels also makes them susceptible to the formation of intermetallic phases from extended exposure to high temperatures. Significant intermetallic precipitation may lead to a loss of corrosion resistance and sometimes to a loss of toughness.
Duplex stainless steels have roughly equal proportions of austenite and ferrite, with ferrite being the matrix. The duplex stainless steels alloying additions are either austenite or ferrite formers. This is occurs by extending the temperature range over which the phase is stable. Among the major alloying elements in duplex stainless steels chromium and molybdenum are ferrite formers, whereas nickel, carbon, nitrogen, and copper are austenite formers.
Composition also plays a major role in the corrosion resistance of duplex stainless steels. Pitting corrosion resistance can be adversely affected. To determine the extent of pitting corrosion resistance offered by the material, a pitting resistance equivalent is commonly used.

Welding Precipitation-Hardenable Stainless Steels

Precipitation-hardening (PH) stainless steels are iron-chromium-nickel alloys. They generally have better corrosion resistance than martensitic stainless steels. The high tensile strengths of the PH stainless steels is due to precipitation hardening of a martensitic or austenitic matrix. Copper, aluminum, titanium, niobium (columbium), and molybdenum are the primary elements added to these stainless steels to promote precipitation hardening.
Precipitation-hardening stainless steels are commonly categorized into three types martensitic, semiaustenitic, and austenitic based on their martensite start and finish (Ms and Mf) temperatures and the resulting microstructures. The issues involved in welding PH steels are different for each group.
It is important to understand the microstructure of the particular type of alloy being welded. Some of the PH stainless steels solidify as primary ferrite and have relatively good resistance to hot cracking. In other PH stainless steels, ferrite is not formed, and it is more difficult to weld these alloys without hot cracking. 

www.smsm.ir

نوع مطلب : مهندسی مواد - متالورژی
نوشته شده در دوشنبه ۱۸ بهمن ۱۳۸۹ توسط SMSM |           |