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Stainless Steels

Stainless Steels are iron-base alloys containing Chromium.  Stainless steels usually contain less than 30% Cr and more than 50% Fe. They attain their stainless characteristics because of the formation of an invisible and adherent chromium-rich oxide surface film. This oxide establishes on the surface and heals itself in the presence of oxygen.  Some other alloying elements added to enhance specific characteristics include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, and nitrogen.  Carbon is usually present in amounts ranging from less than 0.03% to over 1.0% in certain martensitic grades.  Corrosion resistance and mechanical properties are commonly the principal factors in selecting a grade of stainless steel for a given application.

 Stainless steels are commonly divided into five groups:

*        Martensitic stainless steels

*        Ferritic stainless steels

*        Austenitic stainless steels

*        Duplex (ferritic-austenitic) stainless steels

*        Precipitation-hardening stainless steels.

Martensitic stainless steels are essentially alloys of chromium and carbon that possess a martensitic crystal structure in the hardened condition. They are ferromagnetic, hardenable by heat treatments, and are usually less resistant to corrosion than some other grades of stainless steel.  Chromium content usually does not exceed 18%, while carbon content may exceed 1.0 %.  The chromium and carbon contents are adjusted to ensure a martensitic structure after hardening. Excess carbides may be present to enhance wear resistance or as in the case of knife blades, to maintain cutting edges.

Ferritic stainless steels are chromium containing alloys with Ferritic, body centered cubic (bcc) crystal structures. Chromium content is typically less than 30%.  The ferritic stainless steels are ferromagnetic.  They may have good ductility and formability, but high-temperature mechanical properties are relatively inferior to the austenitic stainless steels.  Toughness is limited at low temperatures and in heavy sections. 

Austenitic stainless steels have a austenitic, face centered cubic (fcc) crystal structure. Austenite is formed through the generous use of austenitizing elements such as nickel, manganese, and nitrogen.  Austenitic stainless steels are effectively nonmagnetic in the annealed condition and can be hardened only by cold working.  Some ferromagnetism may be noticed due to cold working or welding.  They typically have reasonable cryogenic and high temperature strength properties. Chromium content typically is in the range of 16 to 26%; nickel content is commonly less than 35%.

Duplex stainless steels are a mixture of bcc ferrite and fcc austenite crystal structures. The percentage each phase is a dependent on the composition and heat treatment. Most Duplex stainless steels are intended to contain around equal amounts of ferrite and austenite phases in the annealed condition. The primary alloying elements are chromium and nickel.  Duplex stainless steels generally have similar corrosion resistance to austenitic alloys except they typically have better stress corrosion cracking resistance.  Duplex stainless steels also generally have greater tensile and yield strengths, but poorer toughness than austenitic stainless steels.

Precipitation hardening stainless steels are chromium-nickel alloys. Precipitation-hardening stainless steels may be either austenitic or martensitic in the annealed condition.  In most cases, precipitation hardening stainless steels attain high strength by precipitation hardening of the martensitic structure.

Selecting a Stainless Steel

There are a large number of stainless steels produced.  Corrosion resistance, physical properties, and mechanical properties are generally among the properties considered when selecting stainless steel for an application.  A more detailed list of selection criteria is listed below:

*        Corrosion resistance

*        Resistance to oxidation and sulfidation

*        Toughness

*        Cryogenic strength

*        Resistance to abrasion and erosion

*        Resistance to galling and seizing

*        Surface finish

*        Magnetic properties

*        Retention of cutting edge

*        Ambient strength

*        Ductility

*        Elevated temperature strength

*        Suitability for intended cleaning procedures

*        Stability of properties in service

*        Thermal conductivity

*        Electrical resistivity

*        Suitability for intended fabrication  techniques

Corrosion resistance is commonly the most significant characteristic of a stainless steel, but can also be the most difficult to assess for a specific application. General corrosion resistance is comparatively easy to determine, but real environments are usually more complex.  An evaluation of other pertinent variables such as fluid velocity, stagnation, turbulence, galvanic couples, welds, crevices, deposits, impurities, variation in temperature, and variation from planned operating chemistry among others issues need to be factored in to selecting the proper stainless steel for a specific environment.

AMC can provide engineering services to determine how to optimize the selection of stainless steel for your application.  Our engineering analysis can reduce overall costs, minimize service problems, and optimize fabrication of your structure.

 

 

 

From NACE2005 Paper 05278 (Corrosion center paper)

Table 2. Chemical analysis of the carbon steels used in the experiments

Element

1020 Composition (%)

X65 Composition (%)

API 5L X65 Standard (%)

AISI 1020 Standard (%)

C

0.19

0.13

< 0.26

0.13-0.23

Mn

0.8

1.16

<1.40

0.30-0.60

P

0.01

0.009

< 0.03

< 0.04

S

0.023

0.009

< 0.03

< 0.05

 

 

Table 3. Hardness (HRB) results

 

1020

X65 longitudinal cut

X65 transversal cut

1

83.5

81.3

60.3

2

84.5

94.4

68.7

3

82.1

98.7

63.3

4

89.1

87.9

78.0

5

83.2

95.4

59.1

6

86.8

89.3

51.1

7

80.9

88.7

66.5

8

80.2

92.9

75.0

9

89.1

93.3

58.5

10

83.2

85.1

67.7

Average

84.3

90.7

64.8

Approx.Tensile Strength

79,000psi for 85HRB

90,000 psi for 90.7HRB

56,000 psi for 65.7HRB

Tensile requirements

58,000 psi (min)

77,000psi (min)

77,000psi (min)

Yield Strength

36,000 psi (min)

65,000psi (min)

65,000psi (min)

 

Chemical composition and mechanical properties of welded carbon steel pipes according:

API-5L and DIN standards

Standard

And

Steel Grade

Chemical composition, %

Tensile strength,

MPa

Yield strength, MPa

Elongation, %

C

Mn

P

S

MAX

MIN

DIN

API

 

 

 

 

 

 

 

St37

 

0.19

 

0.05

0.05

350-480

235

25

 

A

0.21

0.90

0.04

0.05

331

207

 

St44

 

0.23

 

0.05

0.05

430-550

275

21

 

B

0.26

1.15

0.04

0.05

413

241

 

 

X42

0.28

1.25

0.04

0.05

413

289

 

St52

 

0.22

1.7

0.05

0.05

500-650

355

21

 

X52

0.28

1.25

0.04

0.05

455

358

 

Ste385.7tm

 

0.14

1-1.6

0.035

0.025

530-680

385

19

 

X56

0.26

1.35

0.04

0.05

489

386

 

Ste445.7tm

 

0.16

1-1.6

0.035

0.025

560-710

445

18

 

X65

0.26

1.4

0.04

0.05