19. sandvik matr. tech , clemente tallarico

High Alloyed Duplex and Austenitic Stainless Steels. Aspects on Welding

and Fabrication

Clemente Tallarico

Author:

Claes-Ove Pettersson

Peter Stenvall

Zhiliang Zhou

Sandvik Materials Technology

Contents

Introduction

Materials High alloyed Austenitic Stainless Steels

High alloyed Duplex Stainless Steels

Physical and mechanical properties

Machinability and cold working

Welding Joint preparation

Heat input

Shielding and purging gases

Filler metals

Welding Processes

Welding defects

Discussion and conclusions


Introduction

Harry Brearly 1913

Classification of Stainless Steels

Martensitic

Martensitic-Austenitic

Ferritic

Austenitic

Austenitic-Ferritic

Development of new steels is ongoing for, in principle, all five groups


Materials

This presentation focus on Austenitic and Austenitic-Ferritic Stainless Steels

Following examples are chosen:


Properties as a function of alloy content

In general, increasing alloy content => higher corrosion resistance

PRE as a function of alloy content

010

203040

5060

N08904 S31254 S32654

Alloy

PR

E=

Cr+

3.3

xM

o+

16

xN

PRE as a function of alloy content

0

10

20

30

40

50

60

S32205 S32750 S32707

Alloy

PR

E=C

r+3.3

xM

o+16xN



Mechanical properties, Proof strength

Proof strength, 0,2% offset as function of alloy

content, MPa

0

200

400

600

800

S32205 S32750 S32707

N08904 S31254 S32654

Alloy

Pro

of

str

en

gth

,

0,2

%,M

Pa

Austenitic

Austenitic-Ferritic



Elongation, A5

Elongation,A5, as function of alloy

content

0

10

20

30

40

S32205 S32750 S32707

N08904 S31254 S32654

Alloy

Elo

ngat

ion,

A5

%

Austenitic

Austenitic-ferritic


Structure stability

Structure stability decreases with increasing alloy content (Cr, Mo)

Sensitive temperature range

Austenitic, 900 1050C

Austenitic-Ferritic, 800 950C

Austenitic-Ferritic steels are also prone to 475C embrittlement

Influences welding6 0 0

6 5 0

7 0 0

7 5 0

8 0 0

8 5 0

9 0 0

9 5 0

10 0 0

10 5 0

1 10 10 0 10 0 0 10 0 0 0

Time, minT

em

pe

ratu

re,

C

S 3 2 7 5 0

S 3 2 7 0 7


Machinability and Cold working

The machinabilty decreases with increasing alloy content

Austenitic-Ferritic steels somewhat better compared to the

austenitic, eg. N08904 in the

same area and even lower than

S32750

Excessive wear on the rake face of the tungsten carbide bit

Machinability

Corrosion res.

AISI 304

AISI 316

SAF 2205

SAF 2507

Austenitic

Duplex

SAF2707HD


Cold working

More force needed with increasing yield strength (increasing alloy content)

Austenitic-Ferritic steels give more spring back than Austenitic steels at cold forming

A cold work up to approximately 10% can be done without stress relieving (These steels have good resistance to stress corrosion

cracking)

Local heat treatment of the cold worked area is very difficult and not recommended

If heat treatment is necessary, a full quench annealing cycle has to be done (Often difficult in practice)


Welding(Joint preparation)

Compared to 300-series stainless steels, high alloyed duplex and

austenitic steels need a wider

gap and a more open angle due

to the poorer fluidity of the weld

metal


Heat input

Excess heat input shall always be avoided when welding

stainless steels

Austenitic steels more sensitive to excess heat input than

austenitic-ferritic steels

Very important that the interpass temperature is kept


Shielding gas and Purging gas

Nitrogen is an important element

Austenite formation

Corrosion resistance

At TIG welding with pure argon as shielding and root gas,

nitrogen is lost and must be

compensated for in order to

maintain the corrosion resistance


Ferritic area in UNS 32750 weld owing to the loss of nitrogen in fusion line

Shielding gas Ar 99,99%


Filler metal


Filler metal

Highly alloyed austenitic steels are with advantage welded with nickel-based filler metals

Austenitic-ferritic stainless steels are welded with austenitic-ferritic filler metal with the exception of dissimilar joints with

highly alloyed austenitic steels. Then nickel-based fillers have to

be used

Welding austenitic-ferritic steel with nickel-based filler metal gives phenomenons like

Un mixed zones (UMZ)

Nitrogen depleted fusion line


Filler metal

UMZ in root area with sigma Ferritic area in fusion line UMZ in top of weld

Root with normal duplex

structure. Filler 27.9.5.L

Top of weld, normal duplex

structure


Welding processes

The highly alloyed stainless steels are welded by the common arc welding processes

Welding processes giving high heat inputs, eg SAW shall be used with care for the highest alloyed steels


Welding defects

Porosity can occur in nitrogen alloyed steels

At for instance SAW a dry flux is important.

Drying at 370-380C for 4 hours

Too high travel speeds at SAW also promote hot cracking. Aim for an oval shaped weld pool!


Discussion and conclusion

In general, higher alloy content of a stainless steel => improved properties regarding corrosion resistance and mechanical

strength. Problem solvers in severe corrosive environments

Higher alloyed steels => more knowledge needed in fabrication regarding welding, cold working and heat treatment

Close contact with the material supplier recommended


Firstname Surname

Title

Unit, Sandviken Sweden

Thank you for listening!


19. sandvik matr. tech , clemente tallarico

Documents

austenitic steels

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increasing alloy content

series stainless steels

increasing yield strength

cold worked area

local heat treatment

s32750 excessive wear