This document discusses the weldability of new generation stainless steels. It begins by outlining the objectives of comparing the weldability of traditional and newly developed stainless steels. It then provides details on the different types of traditional stainless steels and factors affecting their weldability. The document proceeds to examine the weldability of specific new stainless steel grades, including super duplex, superaustenitic, and supermartensitic steels. It concludes by stating that austenitic stainless steel is the most weldable traditional grade, while newly developed steels can provide better weldability through adjusting welding parameters.

1. 23/07/14 Weldability of New Generation Stainless Steels, JOM-12,M1
Weld-ability of the New GenerationWeld-ability of the New Generation
Stainless SteelsStainless Steels
Hussein Rahmatalla & Yousif Al-HorrHussein Rahmatalla & Yousif Al-Horr
University of QatarUniversity of Qatar
Materials Technology UnitMaterials Technology Unit
Qatar-DohaQatar-Doha

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ObjectivesObjectives
 Metallurgical factors affecting the weld-ability ofMetallurgical factors affecting the weld-ability of
different SS’s.different SS’s.
 Comparison between the weld-ability of theComparison between the weld-ability of the
Traditional SS’s and the newly developed SS’s.Traditional SS’s and the newly developed SS’s.
 Selection of the proper rod or electrode to weldSelection of the proper rod or electrode to weld
the new generation SS’s.the new generation SS’s.

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Stainless Steels (Traditional GradesStainless Steels (Traditional Grades((
 STAINLESS STEELS can be defined as alloys that containSTAINLESS STEELS can be defined as alloys that contain
at least 11% Cr, no more than 0.15% C, and more ironat least 11% Cr, no more than 0.15% C, and more iron
than any other single element. There are five majorthan any other single element. There are five major
families of stainless steels, based on microstructure andfamilies of stainless steels, based on microstructure and
properties:properties:
 Martensitic stainless steelsMartensitic stainless steels
 Ferritic stainless steelsFerritic stainless steels
 Austenitic stainless steelsAustenitic stainless steels
 Precipitation-hardening (PH( stainless steelsPrecipitation-hardening (PH( stainless steels
 Duplex ferritic-austenitic stainless steelsDuplex ferritic-austenitic stainless steels
 Each family requires different weld-ability considerations,Each family requires different weld-ability considerations,
because of the varied phase transformation behavior uponbecause of the varied phase transformation behavior upon
cooling from solidification to room temperature or belowcooling from solidification to room temperature or below

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5. 23/07/14 Weldability of New Generation Stainless Steels, JOM-12,M5
Weldability of the Traditional SS’sWeldability of the Traditional SS’s
 Austenitic is the most weldable. 20-30% less heat input is required than thatAustenitic is the most weldable. 20-30% less heat input is required than that
needed to carbon and low alloy steels, because SS has lower thermalneeded to carbon and low alloy steels, because SS has lower thermal
conductivity and higher electrical resistance.conductivity and higher electrical resistance.
 Austenitic SS is susceptible for micro-fissuring and sensitization in the HAZ.Austenitic SS is susceptible for micro-fissuring and sensitization in the HAZ.
 Ferritic SS, is less weldable, it suffers from, excessive grain growth, lowFerritic SS, is less weldable, it suffers from, excessive grain growth, low
inherent toughness occurred near RT, and precipitation of deleterious phases.inherent toughness occurred near RT, and precipitation of deleterious phases.
 Ferritic SS is limited to be welded in thin sections only (less than 6mm(.Ferritic SS is limited to be welded in thin sections only (less than 6mm(.
Preheating is required for welding thick sections.Preheating is required for welding thick sections.
 Weldability of Martensitic SS is inferior to that of the austenitic, because of theWeldability of Martensitic SS is inferior to that of the austenitic, because of the
phase transformation to martensite which has low fracture toughness value thatphase transformation to martensite which has low fracture toughness value that
leads to hydrogen cracking, when the hydrogen is absorbed.leads to hydrogen cracking, when the hydrogen is absorbed.
 Martensitic SS HAZ region is highly susceptible to cracking.Martensitic SS HAZ region is highly susceptible to cracking.
 Pre-heating and post-heating are always recommended when welding thePre-heating and post-heating are always recommended when welding the
martensitic SS.martensitic SS.

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Weld-ability of Traditional SS’sWeld-ability of Traditional SS’s

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Schaefler (DeLong & WRCSchaefler (DeLong & WRC modifiedmodified((

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Stainless Steels (New GenerationsStainless Steels (New Generations((
 Recently developed SS’s.Recently developed SS’s.
 Lower Carbon and extra alloying elements,Lower Carbon and extra alloying elements,
more Cr, Ni, N, and Mo.more Cr, Ni, N, and Mo.
 Intended to be of better Corrosion Resistance,Intended to be of better Corrosion Resistance,
Weld-ability, or Mechanical Strength.Weld-ability, or Mechanical Strength.
 They are:They are:
- Super Duplex.- Super Duplex.
- Super Austenitic.- Super Austenitic.
- Super Martenstic.- Super Martenstic.

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Duplex and Super-duplex SS’sDuplex and Super-duplex SS’s

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Superaustenitic SS’sSuperaustenitic SS’s
 High Nickel (25-30%( and high chromium. Sometimes comes under nickel baseHigh Nickel (25-30%( and high chromium. Sometimes comes under nickel base
alloys. It contains Mo. It has excellent weld-ability, pitting corrosion resistancealloys. It contains Mo. It has excellent weld-ability, pitting corrosion resistance
and resistance to strong acids.and resistance to strong acids.

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Super-martensitic SS’sSuper-martensitic SS’s New type of weldable martensitic stainless steel grades combine high strength,New type of weldable martensitic stainless steel grades combine high strength,
good corrosion resistance in sweet or mildly sour environments, andgood corrosion resistance in sweet or mildly sour environments, and
acceptable fracture toughness down to - 40 C. A typical microstructure consistsacceptable fracture toughness down to - 40 C. A typical microstructure consists
of low-carbon tempered martensite with high strength and toughness. Someof low-carbon tempered martensite with high strength and toughness. Some
residual finely dispersed austenite is present and little residual ferrite may beresidual finely dispersed austenite is present and little residual ferrite may be
observed in some grades. Typical strength values are:observed in some grades. Typical strength values are:
- 0.2% proof stress: 550-850 MPa- 0.2% proof stress: 550-850 MPa
- Tensile strength: 780 - 1000 MPa- Tensile strength: 780 - 1000 MPa
 In addition to the high strength and low-temperature toughness, it hasIn addition to the high strength and low-temperature toughness, it has
acceptable corrosion resistance in many applications. Apart from the oil andacceptable corrosion resistance in many applications. Apart from the oil and
gas industry other industries such as power generation, chemical processgas industry other industries such as power generation, chemical process
plants, food production and the transportation industry have recognized theplants, food production and the transportation industry have recognized the
benefits of these grade opening up potential fields of application.benefits of these grade opening up potential fields of application.
 A driving force behind this interest is that it can offer an economical choice ofA driving force behind this interest is that it can offer an economical choice of
materials between carbon steels and duplex stainless steels.materials between carbon steels and duplex stainless steels.
 Low Grade: 12Cr, 2Ni.Low Grade: 12Cr, 2Ni.
 Medium Grade: 12Cr, 4.5%Ni, 1.5%Mo.Medium Grade: 12Cr, 4.5%Ni, 1.5%Mo.
 High Grade: 12Cr, 6.5%Ni, 2.5%Mo.High Grade: 12Cr, 6.5%Ni, 2.5%Mo.
 The interest in this type of material is still growing and therefore steelThe interest in this type of material is still growing and therefore steel
manufacturers have put a lot of effort in the further development of martensiticmanufacturers have put a lot of effort in the further development of martensitic
stainless steels with low carbon and nitrogen content and small amounts ofstainless steels with low carbon and nitrogen content and small amounts of
nickel, molybdenum and copper.nickel, molybdenum and copper.

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Weldability of Super-duplex SS’sWeldability of Super-duplex SS’s
 Sensitization:Sensitization: It is less pronounced than in austenitic andIt is less pronounced than in austenitic and
ordinary duplex SS:ordinary duplex SS:
 Less carbon in the Super duplex SS’s, UNS S 32750, contains less thanLess carbon in the Super duplex SS’s, UNS S 32750, contains less than
0.015.0.015.
 In ferrite it is more difficult to form Cr-depleted zone than in austenite.In ferrite it is more difficult to form Cr-depleted zone than in austenite.
 Toughness:Toughness: Higher weld toughness:Higher weld toughness:
 Good toughness could be ascribed to the presence of austenite.Good toughness could be ascribed to the presence of austenite.
 Super DSS is rich in nitrogen (strong austenite stabilizing element).Super DSS is rich in nitrogen (strong austenite stabilizing element).
 Brittle phase; sigma, alpha-prime, and other precipitates contribute toBrittle phase; sigma, alpha-prime, and other precipitates contribute to
the loss of ductility.the loss of ductility.
 Corrosion Resistance of the Weld:Corrosion Resistance of the Weld:
 Less susceptible to nitride formation. Nitrogen facilitates the reformationLess susceptible to nitride formation. Nitrogen facilitates the reformation
of austenite, that in turn, accommodates the majority of nitrogen.of austenite, that in turn, accommodates the majority of nitrogen.
 High heat input and low cooling rates lead to sigma-phase precipitation.High heat input and low cooling rates lead to sigma-phase precipitation.

13. 23/07/14 Weldability of New Generation Stainless Steels, JOM-12,M13
Formation of Brittle phases in UNS S 32750Formation of Brittle phases in UNS S 32750
Temperature-Time-Precipitation TTT diagram for S 32750 indicatingTemperature-Time-Precipitation TTT diagram for S 32750 indicating
Curves for various phase in S 32750 rate of embrittlement at various tempt.Curves for various phase in S 32750 rate of embrittlement at various tempt.

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Electrodes for Welding DSS’sElectrodes for Welding DSS’s

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Weld-ability of the Super-austenitic SSWeld-ability of the Super-austenitic SS
 Successful weld in superaustenitic should have:Successful weld in superaustenitic should have:
-- Weld metal composition of High PREN (Pitting Index)Weld metal composition of High PREN (Pitting Index)
(%Cr + 3.3%Mo + 16%N)(%Cr + 3.3%Mo + 16%N)
-- Microstructure with reduced segregation.Microstructure with reduced segregation.
These can be achieved by using filler metal containing higherThese can be achieved by using filler metal containing higher
Ni, Mo and N than in the base metal: N-enriched, high-Mo, Ni-Ni, Mo and N than in the base metal: N-enriched, high-Mo, Ni-
base alloys are example:base alloys are example:
 Both high Mo and added N are essential for corrosionBoth high Mo and added N are essential for corrosion
resistance.resistance.
 The high Ni content and the added N result in a precipitate-freeThe high Ni content and the added N result in a precipitate-free
entire weld, because of the austenitizing effect.entire weld, because of the austenitizing effect.

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Effect of Ni content on the micro-segregation of Cr and Mo in Cr-Ni-Effect of Ni content on the micro-segregation of Cr and Mo in Cr-Ni-
Fe-Mo alloy welds. (T.Koseki, T.Ogawa, J.Jap. Weld. Soc. 9 (1991), pFe-Mo alloy welds. (T.Koseki, T.Ogawa, J.Jap. Weld. Soc. 9 (1991), p
143143))

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Weld-ability of Super-martensitic SS’sWeld-ability of Super-martensitic SS’s
 Attractive properties regarding strength and corrosion
resistance, but performance details for weldments are
still limited.
 Lower susceptibility to Hydrogen Cracking than the
higher carbon martensitic SS.
 The existence of Ni is to improve the toughness.
 The existence of higher Cr, Mo, and Cu in the base
metal is to improve the resistance to chlorides, CO2
and H2S.
 Recent studies showed that the time required for
PWHT is shorter than that needed for the traditional
martensitic SS.

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Weld-ability of Super-martensitic SS’sWeld-ability of Super-martensitic SS’s
 Available information on filler is the use 13Cr-Available information on filler is the use 13Cr-
4%Ni wires or Super-duplex SS, of very low4%Ni wires or Super-duplex SS, of very low
carbon content (< 0.01%). Both were reportedcarbon content (< 0.01%). Both were reported
to provide acceptable toughness in combinationto provide acceptable toughness in combination
with necessary strength and corrosionwith necessary strength and corrosion
resistance.resistance.
 Chemical composition of the weld metal shouldChemical composition of the weld metal should
be adjusted so as the microstructure is of dualbe adjusted so as the microstructure is of dual
phase M-A or A-F-M.phase M-A or A-F-M.

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ConclusionsConclusions
 Austenitic SS is the most weld-able gradeAustenitic SS is the most weld-able grade
among the traditional SS’s.among the traditional SS’s.
 The new generation SS’s, intended to be ofThe new generation SS’s, intended to be of
higher corrosion resistance, can providehigher corrosion resistance, can provide
better welda-bities, by careful adjustment ofbetter welda-bities, by careful adjustment of
welding parameters.welding parameters.