IIW-AWS

Technical Lectures

The Cr-Mo Steels

January/February 2006

J. F. Henry

The Cr-Mo SteelsA Cornerstone of the Modern Power and

Petrochemical Industries

Lesson 1IIW-AWS

January/February 2006

Topics of Discussion

• History of the Cr-Mo Steels – Lesson 1• Basic Metallurgy of the Cr-Mo Steels – Lesson 2• Welding Issues – Lesson 2• Temper Embrittlement – Lesson 3• Weld-Related Failures – Lesson 4• The “New” Generation of Creep Strength-Enhanced

Ferritic Steels – Lesson 5• Issues of Concern Regarding Control of the “Advanced”

Alloys – Lesson 6

Lesson 1IIW-AWS

January/February 2006

History: 80 Years of Critical Industrial Service

“Oil is King!”Texas Newspaper - 1922

Tosco, Martinez oil refinery

Lesson 1IIW-AWS

January/February 2006

History of Development

• Serious development of alloy steels began in early 1920s

• Development driven by changes in refinery practice

Lesson 1IIW-AWS

January/February 2006

Changes in Refining Requirements

• Increasing demand for gasoline, spurred by mass production of automobiles

• Higher temperatures required for cracking process used to produce gasoline

• Refining temperatures began to exceed the capabilities of carbon steels in terms of both mechanical strength (i.e., creep strength) and corrosion resistance

Lesson 1IIW-AWS

January/February 2006

Corrosion Resistance

• Increasing use of West Texas “Sour” crudes• Excessive corrosion of carbon steels• Initial use of 12Cr & 18Cr/8Ni stainless steels to combat

more aggressive feed stocks• Desire for cheaper materials led to development of 5%Cr

steel in 1928

Lesson 1IIW-AWS

January/February 2006

Elevated Temperature Strength

• At higher processing temperatures required for new cracking operations, carbon steel inadequate

• Discovery that additions of molybdenum (Mo) or tungsten (W) substantially improved elevated temperature performance

• Addition of Mo and/or W to 5Cr steel offered better elevated temperature strength (compared to CS) in an alloy that was more resistant to sulfur-based corrosion than CS

• Mo became the alloy addition of choice – lower cost and improved resistance to temper embrittlement

Lesson 1IIW-AWS

January/February 2006

Milestones In Alloy Development

Lesson 1IIW-AWS

January/February 2006

Developing New Alloys

• Variations in the composition of refinery feed stock, particularly sulfur content of crude, led to demand for alloy “tailoring”

• For “sweet” crudes (lower S), lower Cr content needed for corrosion resistance, and improved creep strength obtained through additions of Mo and, in some cases, vanadium (V). This led to development of “leaner” alloys:a) 1-3%Cr with Mo or Wb) 2Cr-1/2Mo for resistance to graphitization

Lesson 1IIW-AWS

January/February 2006

By the mid 1930’s the

basic group of Cr-Mo steels

had been approved by

ASTM/ASME

Lesson 1IIW-AWS

January/February 2006

Evolution of Alloy Development

Piercing Mill Making A Seamless Tube Of 7%Cr Steel

(Timken Roller Bearing Co.)

Lesson 1IIW-AWS

January/February 2006

Evolution of Alloy Development

• Refining of increasing amounts of “sour” crude led to experimentation with Cr contents >5%, but <12% needed for stainless steels

• 7-9% Cr-Mo steels developed• For refining applications improvement in corrosion

resistance was significant:5Cr steels 4-10X more corrosion resistant than CS7Cr steels 2X more corrosion resistant than 5Cr steels9Cr steels 4X more corrosion resistant than 5Cr steels

Lesson 1IIW-AWS

January/February 2006

Evolution of Alloy Development

• For critical bolting applications at lower temperatures, high tensile strength desired

• Materials such as 4140 – CrMo alloy with 0.40% C – were developed to facilitate through hardening

Lesson 1IIW-AWS

January/February 2006

Cr-Mo Steels - Carbon Content

• For many of the most widely-used Cr-Mo alloys, the carbon content is maintained < 0.15% (weight)

• Higher C adversely affects weldability• Higher C does not substantially improve creep strength• Higher C can reduce corrosion resistance through excess

carbide formation

Lesson 1IIW-AWS

January/February 2006

Cr-Mo Steels in the Power Industry

In the Power Industry, the introduction of welding for major pressure parts spurred rapid increase in

steam temperatures and pressures

Lesson 1IIW-AWS

January/February 2006

• At steam outlet temperatures >400°C (750°F) there was a need for alloys with higher creep strength and improved oxidation resistance

• The Cr-Mo steels developed for the petrochemical industry proved to be a good fit

Cr-Mo Steels in the Power Industry

Lesson 1IIW-AWS

January/February 2006

Development of “Advanced” Ferritic Steels

• Work in the 1930s and 1940s had demonstrated the potent strengthening effect of small additions of V, Ti, Cb, and other carbide forming alloy additions

• The successful introduction of X20CrMoV, a higher carbon vanadium fortified 12Cr alloy, in Europe in the early 1960s was an early result of these efforts

• Further advances were made in support of the nuclear industry’s fast breeder reactor programs beginning in the early 1970s

• These efforts culminated in the development of the modified 9Cr-1Mo alloy, now known as Grade 91

Lesson 1IIW-AWS

January/February 2006

Creep Rupture Strength

Lesson 1IIW-AWS

January/February 2006

Creep Strength Enhanced Alloys

• Following cancellation of the fast breeder programs in the late 1970s, application of Grade 91 and later variants (i.e., Grades 92, 911, 122, 23, etc.) shifted to the fossil power industry

• Favored by designers for improved performance in cyclic service (conventional boilers and HRSGs)

• An essential component of the materials strategy for ultra-supercritical boilers

Lesson 1IIW-AWS

January/February 2006

0

5

10

15

20

25

800 850 900 950 1000 1050 1100 1150

Temperature (F)

Allo

wab

le S

tress

(ksi

)

P22

P91

• Design Codes do not penalize for increased thickness.• Thermal stress varies with square of thickness.• Fatigue life varies with cube of stress.

• High creep-strength materials (e.g.,P91) reduce wall thickness in high temperature areas, which improves cycling capability.

• P91 gives 40% thickness reduction for same creep life and 12 times the fatigue life.

Design Interest In CSEF Steels