weld ability of the new generation stainless steels

10/04/2310/04/23 Weldability of New Generation StWeldability of New Generation Stainless Steels, JOM-12,March 20-ainless Steels, JOM-12,March 20-23, 2005 Helsingor,Denmark 23, 2005 Helsingor,Denmark

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Weld-ability of the New Weld-ability of the New Generation Stainless SteelsGeneration Stainless Steels

Hussein Rahmatalla & Yousif Al-HorrHussein Rahmatalla & Yousif Al-Horr

University of QatarUniversity of QatarMaterials Technology UnitMaterials Technology Unit

Qatar-DohaQatar-Doha


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ObjectivesObjectives

Metallurgical factors affecting the weld-ability of Metallurgical factors affecting the weld-ability of different SS’s.different SS’s.

Comparison between the weld-ability of the Comparison 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 weld Selection 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 Stainless Steels (Traditional Grades)Grades) STAINLESS STEELS can be defined as alloys that contain STAINLESS STEELS can be defined as alloys that contain

at least 11% Cr, no more than 0.15% C, and more iron at least 11% Cr, no more than 0.15% C, and more iron than any other single element. There are five major families than any other single element. There are five major families of stainless steels, based on microstructure and properties: of stainless steels, based on microstructure and 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 upon because 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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Weldability of the Traditional SS’sWeldability of the Traditional SS’s

Austenitic is the most weldable. 20-30% less heat input is required than that Austenitic is the most weldable. 20-30% less heat input is required than that needed to carbon and low alloy steels, because SS has lower thermal needed 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, low Ferritic 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 the Weldability of Martensitic SS is inferior to that of the austenitic, because of the

phase transformation to martensite which has low fracture toughness value that phase 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 the Pre-heating and post-heating are always recommended when welding the

martensitic SS. martensitic SS.


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


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Schaefler (DeLong & WRCSchaefler (DeLong & WRC modified)modified)


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Stainless Steels (New Stainless Steels (New Generations)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 Duplex and Super-duplex SS’sSS’s


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Superaustenitic SS’sSuperaustenitic SS’s High Nickel (25-30%) and high chromium. Sometimes comes under nickel base High Nickel (25-30%) and high chromium. Sometimes comes under nickel base

alloys. It contains Mo. It has excellent weld-ability, pitting corrosion resistance alloys. 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, and good corrosion resistance in sweet or mildly sour environments, and acceptable fracture toughness down to - 40 C. A typical microstructure consists acceptable fracture toughness down to - 40 C. A typical microstructure consists of low-carbon tempered martensite with high strength and toughness. Some of low-carbon tempered martensite with high strength and toughness. Some residual finely dispersed austenite is present and little residual ferrite may be residual 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 has In addition to the high strength and low-temperature toughness, it has acceptable corrosion resistance in many applications. Apart from the oil and acceptable corrosion resistance in many applications. Apart from the oil and gas industry other industries such as power generation, chemical process gas industry other industries such as power generation, chemical process plants, food production and the transportation industry have recognized the plants, 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 of A 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 steel The interest in this type of material is still growing and therefore steel

manufacturers have put a lot of effort in the further development of martensitic manufacturers have put a lot of effort in the further development of martensitic stainless steels with low carbon and nitrogen content and small amounts of stainless 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 and It 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 than Less 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 to Brittle 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 reformation Less 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.


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Formation of Brittle phases in Formation of Brittle phases in UNS S 32750UNS S 32750

Temperature-Time-Precipitation TTT diagram for S 32750 indicating Temperature-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-Weld-ability of the Super-austenitic SSaustenitic 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 higher These 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 corrosion Both high Mo and added N are essential for corrosion resistance.resistance.

The high Ni content and the added N result in a precipitate-free The 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), p Fe-Mo alloy welds. (T.Koseki, T.Ogawa, J.Jap. Weld. Soc. 9 (1991), p 143)143)


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Weld-ability of Super-martensitic Weld-ability of Super-martensitic SS’sSS’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 Weld-ability of Super-martensitic SS’sSS’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 low 4%Ni wires or Super-duplex SS, of very low carbon content (< 0.01%). Both were reported carbon content (< 0.01%). Both were reported to provide acceptable toughness in combination to provide acceptable toughness in combination with necessary strength and corrosion with necessary strength and corrosion resistance.resistance.

Chemical composition of the weld metal should Chemical composition of the weld metal should be adjusted so as the microstructure is of dual be 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 grade Austenitic 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 of The new generation SS’s, intended to be of higher corrosion resistance, can provide higher corrosion resistance, can provide better welda-bities, by careful adjustment of better welda-bities, by careful adjustment of welding parameters. welding parameters.

weld ability of the new generation stainless steels

Engineering

containstainless steels

new generation sss

low alloy steels

traditional sss austenitic

austenitic ss

new generationweldability

different sss

m1 weldability