AWS D10.18M/D10.18 pdf free download

AWS D10.18M/D10.18 pdf free download.Welding Ferritic/Austenitic Duplex Stainless Steel Piping and Tubing.
5.2 Precipitating Phases. As noted above, after the molten metal solidifies as ferrite, some ferrite transforms to austenite down to a temperature of about l(XX)°C 11 832°F1. As the DSS cools below this temperature, other phases such as carhides, nitrides. sigma, and other intermetallic phases can form. Chi phase and other intermetallic phases are often lumped together with sigma phase, and the term “sigma phase” will be taken hereafter to include these other intermetallie phases. Sigma, in significant amounts, can be particularly detrimental in that toughness and corrosion resistance may be reduced. Sigma forms in the temperature range of 700°C to 980°C 11300°F to I 8(X)°F] with the most rapid formation at 870°C 11600°F]. It is formed whenever the alloy is in the 700°C to 980°C [1300°F to l800°F1 temperature range, being there is an accumulative effect. The total exposure time for sigma formation includes time at the steel mill during in-process heating and exposure time during welding including repair welding unless there is an intervening anneal above 1040°C 11 9(X)°F1. The proper level of nitrogen delays sigma formation, hut does not prevent it. It has been shown that in the case of alloy 2205 with nitrogen in the upper range of 0.14% to 0.20%, there is about a five-minute exposure time at 870°C [1600°F] available before the amount of sigma formed causes a significant loss of toughness and corrosion resistance.
As time at temperature in processing the wrought products is equally as important as time incurred during welding, ASTM A923. Standard Test MetI,odsfrr Detecting Detrimental I,,tern,erallic Phase in Duplex Ausrenitic/Ferritic Stain— less Steels, is a very useful document to utilize as it describes three test methods for detecting detrimental intermetallic phases. The methods include illustrations of acceptable microstructures, Charpy impact test acceptance criteria, and a ferric chloride corrosion test. ASTM A923 can he specified as a procurement requirement and is particularly useful for the thicker product forms. It is an aid in assuring that a significant portion of the allowable “five minutes at sigma formalion temperature” has not been used at the steel mill, allowing less lime at temperature during welding. ASTM A 1084 is a standard for the lean DSS for detecting detrimental intermetallic phases and should be used in place of ASTM A923 for the lean DSS.
With nitrogen present in the duplex stainless steels, there is a potential to form chromium nitndes in the interior of large ferrite grains in the heat-affected zone (HAZ) that can result from too rapid cooling and/or inappropriate alloy balance. Large amounts of chromium nitrides could create chromium depleted areas that if not corrected by annealing could reduce corrosion resistance. However, the higher level of nitrogen such as (0.14% to 0.20%) found iii 2205 (UNS S32205) promotes austenite which has a much higher soluhility of nitrogen. resulting in the elimination of chromium nitrides in the second-generation duplex stainless steels. Second-generation alloys are also made to very low carbon levels, reducing the concern for carbide formation.
Alpha prime is an embrittling phase that forms in ferritic stainless steels with over 16% chromium, as well as in the ferrite phase of DSS. when exposed in the temperature range of 400°C to 540°C 1750°F to 1000°F] with peak embrittling occurring at 475°C 1885°F]. The phenomena is commonly referred to as 475°C [885°F] embritilement and results in a severe loss of room temperature ductility and toughness. The Lime to embrittle at 475°C 1885°F] can vary from a few minutes for a DSS with a low level of nitrogen and large amount of ferrite to several hours for alloys with lower ferrite and higher nitrogen. It is the concern for alpha prime embrittlement that ASME limits DSS to an upper service temperature limit of 3 15°C 1600°F] while other standards such as some European codes limit welded alloy 2205 to 250°C [480°FI.
The time and temperature to form the phases discussed above, i.e., sigma, carbides, nitrides. and alpha prime is illustrated in Figure 2 for 2205 that was annealed at 1050°C [1920°F]. The ‘nose” of the lines is the start of the particular phase formation with transformation continuing for a period of time. The time tO reach a deleterious level may or may not be reached during the time. Alloys 2304 and 2507 are shown for comparison. Significant points to note are:
(I) While chromium carbides and nitrides start to form in 1—2 minutes, this is actually skwer than in the ferritic and highly alloyed austenitic grades and as noted earlier these phases are seldom of a practical concern.
(2) Sigma and chi formation takes place at a somewhat higher temperature, hut at about the same time as carbides and nitrides. It will be noted that transformation of the higher alloyed 2507 initiates in less time and the leaner alloy 2304 takes considerably longer time than 2205.AWS D10.18M/D10.18 pdf download.