WELDABILITY OF CR-MO STEELS

WHAT ARE CR-MO STEELS??

Steels containing 0.5-9% Cr and 0.5-1% Mo are referred to as Cr-mo steels.

The carbon content in such steels varies in between 0.10 to 0.20%.

Because these steels contain very low carbon hence they are weldable.

Successful welding of Cr-Mo steels re- quires proper design, material selection, and quality control throughout all phases of engineering and construction.

SAE grade

 % Cr  % Mo  % C *  % Mn  % P (max)  % S (max)  % Si

4118 0.40–0.60 0.08–0.15 0.18–0.23 0.70–0.90 0.035 0.040 0.15–0.35

4120 0.40–0.60 0.13–0.20 0.18–0.23 0.90–1.20 0.035 0.040 0.15–0.35

4121 0.45–0.65 0.20–0.30 0.18–0.23 0.75–1.00 0.035 0.040 0.15–0.35

4130 0.80–1.10 0.15–0.25 0.28–0.33 0.40–0.60 0.035 0.040 0.15–0.35

4135 0.80–1.10 0.15–0.25 0.33–0.38 0.70–0.90 0.035 0.040 0.15–0.35

4137 0.80–1.10 0.15–0.25 0.35–0.40 0.70–0.90 0.035 0.040 0.15–0.35

4140 0.80–1.10 0.15–0.25 0.38–0.43 0.75–1.00 0.035 0.040 0.15–0.35

4142 0.80–1.10 0.15–0.25 0.40–0.45 0.75–1.00 0.035 0.040 0.15–0.35

4145 0.80–1.10 0.15–0.25 0.43–0.48 0.75–1.00 0.035 0.040 0.15–0.35

4147 0.80–1.10 0.15–0.25 0.45–0.50 0.75–1.00 0.035 0.040 0.15–0.35

4150 0.80–1.10 0.15–0.25 0.48–0.53 0.75–1.00 0.035 0.040 0.15–0.35

4161 0.70–0.90 0.25–0.35 0.56–0.64 0.75–1.00 0.035 0.040 0.15–0.35

* The carbon composition of the alloy is denoted by the last two digits of the SAE specification number, in hundredths of a percent

Material Condition Tensile strength [psi (MPa)]

Yield strength [psi (MPa)]

Elongation in 2" [%]

Hardness (Rockwell)

4130 Cold drawn—normalized

85,000–110,000 psi (590–760 MPa)

70,000–85,000 psi (480–590 MPa) 20–30 B 90–96

4142

Hot rolled—annealed

90,000–100,000 psi (620–690 MPa)

60,000–70,000 psi (410–480 MPa) 20–30 B 90–95

Cold drawn—annealed

105,000–120,000 psi (720–830 MPa)

85,000–95,000 psi (590–660 MPa) 15–25 B 96–100

4150 Hot rolled—annealed

90,000–110,000 psi (620–760 MPa)

65,000–75,000 psi (450–520 MPa) 20–30 B 90–96

Mechanical properties

PROPERTIES OF CR-MO STEELS

High harden-ability. Good oxidation and corrosion resistance

at elevated temperature. Good creep strength(best in 2.25 Cr-1

Mo). Do not become brittle even after extended

elevated temperature service. Ability to be case hardened by

carburization.

APPLICATIONS OF CR-MO STEELS

Petroleum industry

Elevated temperature applications

Air-craft tubings

WELDING OF CR-MO MATERIALS, PRE-1950’S

Low-hydrogen shielded metal arc welding (SMAW) electrodes were not manufactured at that time; therefore, cellulose-covered electrodes such as E7010, E8010, and E9011 containing additions of Cr and Mo were used.

Weld-ability was poor compared with modern standards.

Repairs were especially prone to cracking, due to the localized heating and the relatively high hydrogen content of the cellulose-coated electrodes.

After the first decade of Cr-Mo welding, three major considerations were identified:

•Hydrogen contamination • Temper embrittlement • Stress concentrations

Recognition of these common factors, which often led to cracking during welding of Cr-Mo steels prompted research to improve the welding processes.

LIMITING HYDROGEN CONTAMINATION

Uncontrolled exposure of electrode to the atmosphere can lead to hydrogen absorption by the flux that could be introduced into the weld.

E8018-B2 electrodes, used in conjunction with ovens, introduce minimal hydrogen into a weld.

The low-hydrogen electrodes coupled with preheat techniques vastly improved the weld-ability, while lowering the risk of delayed (hydrogen-induced) cracking.

Pre-heat eliminates hydrogen sources, such as condensate, from the material surface and slows the cooling rates giving entrapped hydrogen time to diffuse from the weld.

Post heat, that is holding the weldment at inter-pass temperatures after welding provides additional diffusion time for hydrogen to escape.

The PWHT, standardized at 677°C for 1 h/in. of plate thickness provides stress relief of the vessel lowers the weld metal hardness, and allows more time for the hydrogen to diffuse from the weld metal.

The PWHT should not exceed the tempering temperature applied by the steel mill. Exceeding the tempering temperature degrades the mechanical properties of the base material.

Retaining mechanical properties after several PWHT operations is difficult for both the base materials and the welds.

PRE-HEAT TEMPERATURES

STEEL UPTO 13mm 13-25 mm OVER 25 mm

0.5 cr-0.5 mo 38c 95c 150c

1 cr-0.5 mo 38c 95c 150c

1.25 cr-0.5 mo 38c 95c 150c

2 cr-0.5 mo 65c 95c 150c

2.25 cr-1 mo 65c 95c 150c

3 cr-1 mo 120c 150c 205c

5 cr-0.5 mo 205c 205c 260c

7 cr-0.5 mo 205c 205c 260c

9 cr-1 mo 205c 205c 260c

PWHT TEMPERATURES

STEEL TEMPERATURES

0.5 cr-0.5 mo 635-705C

1 cr-0.5 mo 635-730C

1.25 cr-0.5 mo 635-730C

2 cr-0.5 mo 635-730C

2.25 cr-1 mo 675-745C

3 cr-1 mo 675-745C

5 cr-0.5 mo 675-760C

7 cr-0.5 mo 675-760C

9 cr-1 mo 675-760C

CONTROLLING TEMPER EMBRITTLEMENT

Temper embrittlement is defined as a decrease in toughness when the material is heated or cooled through the 300°–600°C temperature range.

The un-intentional additions of silicon, phosphorus, tin, antimony, and arsenic can increase the susceptibility to temper embrittlement.

Temper embrittlement factor, X = 10 ⋅ P + 5 ⋅ Sb + 4 ⋅ Sn + As  100

The accepted limits are X ≤ 15; however, critical applications of higher-alloyed Cr-Mo materials may require X ≤ 12.

MINIMIZING STRESSCONCENTRATIONS

 

Stress concentrations are responsible for a variety of crack-related failures and must be minimized by design.

All welds must be profiled to eliminate sharp transitions and excessive reinforcement.

Grinding should be done with care to produce smooth parts.

WELDING PROCESSES FOR CR-MO STEELS

SMAW,TIG,MIG,SAW,FCAW,ESW,EBW,LASER WELDING,FRW,RSW,BRAZING can be used to weld Cr-Mo steels.

Filler metal of the same or slightly higher alloy content can be used for welding several Cr-Mo steels.

For eg;1.25 Cr-0.5 Mo filler can be used for welding 0.5Cr-0.5 Mo and 1.25 Cr-0.5 Mo.

Electrodes used must be of low hydrogen specifications.

309-310 grades of s.s are often employed for minor repair welding of Cr-Mo steels.

However these grades are not used in applications involving thermal cyclic stresses because the difference in co-efficients of thermal expansions results in internal stress at weld interface during service.

REFERANCES Stewart, C. W., Stryk, A., and Pres- ley, L.

2006. Coke drum design. Petroleum Technology Quarterly, Q3.

ASM Metals Handbook, Vol. 1, Properties and Selection. 2000. Materials Park, Ohio: ASM International, p. 689.

Bruscato, R. 1970. Temper embrittlement and creep embrittlement of 2¼ Cr-1 Mo shielded metal arc weld deposits. Welding Journal 49(4): 148–156

Welding engineering & technology by R.S parmar.

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