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Techniques for Joining1¼Cr-½Mo Steels

 Fig. 1 — Fluid catalytic cracking unit, Turnaround, Baton Rouge, La., 1999.

BY JAMES BRENNA

AND BEN PLETCHE

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standard practices such as API 934-C, Materials and Fabrication of 1¼ C-½ MoSteel Heavy Wall Pressure Vessels for High- Pressure Hydrogen Service Operating at or  below 825°F (441°C).

Applications ASTM SA-387, Grade 11 defines the

1¼ Cr-½ Mo steels. Minor additions of the elements Cr and Mo, to standard car-bon steels, provide creep resistance at el-evated service temperatures. Typical platestructures, utilizing SA-387, Grade 11 ma-terial, include reactor vessels and cokedrums for refinery operations (Fig. 1), and

basic oxygen furnaces for steel mills.These large structures have wall thick-nesses from 1 to 3 in. (25 to 75 mm), are22–36 ft (6.7–11 m) in diameter, and be-tween 80 and 120 ft (24 and 36 m) high.Coke drums endure the most severe cycli-cal thermal service with temperaturesreaching as high as 1000°F (538°C) fol-lowed by a water quench. Typical serviceallows for two thermal cycles per day and

an estimated vessel lifespan of 3000 cy-cles. Figure 2 shows an extreme exampleof a coke drum replaced after seven yearsin service. Bulging of the vessel was causedby thermal fatigue exacerbated by the dif-ferential yield strength between the basemetal and weld. Higher-strength weld

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mill. Exceeding the tempering temture may degrade the mechanical prties of the base material.

Controlling TemperEmbrittlement

Base material and electrode chemplay a major role in both vessel structibility and its service applicaTemper embrittlement is defined as crease in toughness when the materheated or cooled through the 570°–1(300°–600°C) temperature range (ReThis temperature range coincides witthermal cycling that a coke drum exences in service. In addition, the unitional additions of silicon, phosphoruantimony, and arsenic can increase thceptibility to temper embrittlement.

In the late 1960s, CB&I reseaRobert Bruscato examined Cr-Modeposits and quantified the relationbetween these trace impurities andeffects on temper embrittlement. Thcommon Bruscato temper embrittlefactor, X, can be calculated using thlowing formula, where elements are in parts per million (ppm) (Ref. 3).

 X P Sb Sn As

=⋅ + ⋅ + ⋅ +10 5 4

100

 Fig. 3 — A vertical plate coke drum in fabrication.

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rior weld joints, especially girth joints, toa flush, smooth and blended finish is com-mon practice in coke drum fabrication.

1¼ Cr-½ Mo WeldingChallenges

 Advancements in the processes usedfor manufacturing welding consumables,driven by the need to limit trace elementsand lower diffusible hydrogen, have giventhe industry many good options for weld-ing Cr-Mo materials. Most SMAW con-sumables can easily meet the X ≤ 15 re-quirements, while providing excellent

 weldability. Modern submerged arc weld-ing (SAW) electrodes also have excellentchemical makeup. When used in conjunc-tion with significantly improved SAWfluxes, these consumables result in mini-mal cracking, smooth weld beads, andgood operator appeal. The AC waveformcontrol used in SAW is a recent modifica-tion to the process. This AC waveformtechnology allows greater control of thedepth of penetration and deposition rates,

 while using the same consumable/flux combination. These techniques are easilyapplied to shop-built vessels decreasingproduction time. While significant im-provements have already created good

consumables for SMAW and SAW, flux cored arc welding (FCAW) remains a newproduct to the market. History shows thatcareful material evaluations are requiredto ensure that all design requirements aremet by any new product.

Preheat and postheat methods are al- ways being pushed for faster application while providing improved energy efficiency.Heating rates and temperature control canbe improved with modern gas burners andelectric resistance heaters formed to fit the

 vessel shell — Fig. 4. As previously noted,PWHT must consider the temper of theplate material, as surpassing this tempera-ture may have detrimental effects on the

mechanical properties. Owners and endusers often require multiple thermal cycles,allowing for subsequent repairs followedby PWHT, thereby increasing the life of the

 vessel. Retaining mechanical propertiesafter several PWHT operations is difficultfor both the base materials and the welds.Ultimately, the vessel is limited by the ma-terial to which it was designed and the con-ditions of its operation.

Summary

Significant advancements over thepast half-century have made 1¼ Cr-½Mo

steels readily weldable. Ongoing resis pushing to increase productivienhancing the deposition in

 waveform-control led SAW and FCAW consumables. Material and dimprovements are continually push

the desire for steel plate structureslonger lifespans. Innovative designs,as the vertical plate coke drum, prfundamental engineering improvem

 while quality consumables limitintroduction of detrimental elemWhatever the future holds, three factors must be controlled to proquality products: hydrogen contamtion, temper embrittlement, and s

concentrations.N

 References

1. Stewart, C. W., Stryk, A., and ley, L. 2006. Coke drum design. Petro

Technology Quarterly, Q3.2. ASM Metals Handbook, Vol. 1,

erties and Selection. 2000. Materials

Ohio: ASM International, p. 689.3. Bruscato, R. 1970. Temper em

tlement and creep embrittlement oCr-1 Mo shielded metal arc weld depWelding Journal 49(4): 148–156.