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Valve World 2002 2002 KCI Publishing BV

Introduction to valves in the new high chrome alloy8Cr-1Mo-V for high pressure/temperatureapplications

Authors:A.K. Velan, M. Mech. Eng., Prof E.CEO & President of Velan ValveCorporation, Canada

Quality of castings impairedby metal flow andsolidification problems

In spite of the enormousimprovements in foundry technology,assisted by computerized processesto optimize the design of patternsand flow geometry during pouring itis extremely difficult to achieve X-Ray / MT or PT quality to Class 1 or2 acceptance standards required inHP/HT applications in Nuclear,Thermal Power Stations or severeservices in Petrochemical Industries.Upgrading by welding is required.

After weld repairs, however, theoverall quality and reliability of a castvalve is difficult to determine.Sometimes all that is left is askeleton casting with weld metal.

Test bars are usually cast for eachheat, but their analysis may beinconclusive. Even if the round testbar indicates acceptable chemicaland physical properties, the castingitself may have undetectableinherent deficiencies that reducestrength or corrosion resistance.

The periodic in-service inspection ofSection IX of the Boiler Code includesweld repairs in cast metal, and pipewelds. Records of weld-repairlocations must therefore bemaintained, so signs of distress foundduring plant operation can be relatedto the original fabricated conditionsand standards.

Metal poured into a mold cavityduring the casting process solidifieswith the possibility of shrinkage,segregation or porosity, which can

prevent an as-poured casting frombeing acceptable for severeapplications. Shrinkage occurs in twoparts - as metal superheats above themelting point and has losses, causingshrinkage, followed by furthershrinkage during solidification. Addedmelt compensates for the first part,but the contraction during cooling inthe solid state must be compensatedby oversized dimensions.

Segregation, or chemical separationof the melt, occurs duringsolidification when a layer forms atthe mold-cavity wall and progressesinward. Low fluidity over a widetemperature range allows small solidcrystalsdendritesto form and growin a treelike pattern. The initialcrystals, freezing next to the moldwalls, have the lowest alloy content.Farther in, the core has high alloyconcentration, bearing littleresemblance to the intendedcomposition. Within each dendritearm, microsegregation occurs. Resultis microporosity, second-phaseprecipitation and inclusions of

intermetallic or non-metalliccompounds. Porosity can be causedby gases coming out of solutionduring cooling and becoming trappedbetween dendrite arms as tiny voids.Also, as dendrites solidify and shrinkin volume, replacement of melt mustflow along a tortuous path ofinterleaving dendrites. Resistance toflow may be high enough to causemicrovoids and porosity.

Some other defects in castings arewell-defined cracks and hot tears that

develop during solidification, undercombination of stress concentrationfrom uneven contraction and themetals low strength at near-meltingtemperatures. Cold shots develop

from low casting temperature, and dirtspots result from pickup of sand orslag by molten metal. Poor foundrypractices can cause otherdeficiencies, too.Upgrading of castings to meet X-Rayquality needs relies on the grindingout of faulty areas, weld repairs, heattreatment, and retest andexamination. Even then, seating andgasket faces or buttweld ends canshow fine-line cracks that need moreupgrading by rewelding andremachining.

Quality of the new high alloycastings 9Cr-Mo-V is evenmore difficult to achieve

New alloy 9Cr1MoV

As power plants pressures (up to4100 psi) and temperatures (up to1100F (593C) are being steadilyraised to increase efficiency a newalloy 9Cr1MoV with higher creepstrength then F22 or even SS316 has

been introduced for pipes, valves andfittings. Large savings can beachieved in piping because ofreduced wall thickness. The ASMEallowable stresses are shown inTable 1.

Table 1: ASME code section II

allowable stresses(1,000 psi).

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Castings in 9Cr1MoV

have not been yet (June 2001)assigned a grade by the ASME. Thecastings are ordered to the ASMEcode case 2192-21. The ASTMgrade is A217 C12A.

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Difficult processing issues in

pouring castings

The excellent high-temperatureproperties of 9CR1MO-V alloysdepend on the formation of aparticular microstructure (Figure 1)containing submicroscopic carbides.This is affected by the deoxidisation

practices, heat-treatment, thewelding process and PWHT. If thoseprocesses are not properly executed,the required creep properties will notbe realized even though thecomposition and room temperaturemechanical properties will appearacceptable.(1)

Deoxidisation

To remove dissolved oxygen in themolten metal prior to pouring,elements have been added whichhave high affinity for oxygen such astitanium, vanadium, niobium andmagnesium. The proper processing,which is difficult, can assure the finalacceptable proportion microstructure(Figure 1) and the expected high-temperature creep strength.

Heat treatment

The HT procedure is also critical andrequires strict controls to ensure theproper microstructure.

Difficulties in upgrading of

castings by welding to meet

X-Ray quality(1)

The high chrome content (9%)provides very high hardenability.Consequently martensite will form inthe welds and heat affected zone(HAZ) requiring proper and precisePWHT procedures. The properinterpass temperature is required toprevent cold and hot crackingas well as the proper selection of

welding filler material, to produce theproper creep resistance andtoughness.

Conclusion

1. During the critical deoxidisationthe high affinity elements canalso combine with nitrogen andprevent during the heattreatment the formation of fineniobium carbonitrides which areessential for the hightemperature properties.(1)

2. If the material is not properlymelted, heat treated, weldedduring X-Ray upgrading orduring manufacturing and

PWHT the casting will lack thehigh temperature properties.(1)

As it is impractical to obtain amicrostructure for each casting thereliability of a given valve body isquestionable, which may result in acatastrophic failure after lengthyservice in high temperature

(950-1100F) applications.

Acceptable microstructure of castingswithout ferrites or blocky carbides(100X)(1)

Unacceptable microstructure incastings with large white ferrite areas.(200X)(1)

Quality of forging begins

with the segregation-free,uniform and pure ingot

Special alloys and stainless steels aremanufactured in vacuum inductionmelt furnaces. Electrodes produced inthese furnaces are remelted invacuum with electrodes presseddirectly from chemical powders,resulting in further purification.Solidification is controlled byelectronic circuits making possiblesegregation-free ingots a taskimpossible to achieve with castings.

Alloy steel heats are vacuum-treatedon degassing units to reducehydrogen contact to less than twoparts per million and reduce metallicoxides. Teeming (pouring) of ingots isdone usually under argon protection.

All processes proceed under thewatchful eyes of the laboratoriesusing sophisticated equipment.

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Advanced forging processfor Velan 12-24 bodyforging

The plus value of forging forHP/HT valves

Why forging?

When compared with castings, forgedvalve bodies offer the advantages ofmore uniform structure, greaterdensity, higher strength integrity,enhanced dimensional characteristicsand closer dimensional tolerances.The directional structure (flowlines) issuperior from an overall strength____________________________________

High strength

Hot forging promote recrystallizationand grain refinement allowing thematerial to develop maximumpossible strength and uniformity witha minimum variation from piece topiece. The grain flow closely followsthe outline of the body andcontinuous flow lines decrease the

susceptibility for fatigue or commonfailures. (Figure 2)

Structural integrity

Forging eliminates internal flaws andproduces a coherent and uniformmetallurgical structure assuringoptimum performance. Where stressand intragranular corrosion are aproblem, a forging will assure long lifeand troublefree service.

Reliability

The ability of forging to meet designrequirements consistently is one ofthe most important advantages andtakes into account all the precedingcharacteristics to some degree.

Dimensional and metallurgical

uniformity

Dimensional uniformity of closed-dieforging results in positive control ofcritical wall thickness, eliminating

deficiencies caused by shifted coresin castings. A uniform metallurgicalstructure without internal flaws isassured by (a) quality, segregation-free billet and (b) high impact forces

achieved on 10,000 30,000 tonpresses.

Qualityassurance of forging

Through the use of forging, with theiruniformity and high quality, the

radiographic requirement forcomparable Class 1-cast componentsis eliminated. The same attitude hasbeen taken by the United States Navywhen using forging for valves andothercomponents for nuclear submarinesand aircraft carriers. All that isrequired by the ASME Code isultrasonic examination and magneticparticle or liquid penetrant testing inthe finished condition. Rejections offorging for inherent deficiencies foundby U.T., M.T., or P.T. methods are

rare. Procurement of parts, lead timescan be controlled, resulting in morereliable valve deliveries.

Comparison of directional

structure of cast & forged

bodies

Figure 1: Continuous flow lines inhighly stressed crotcharea.

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Fatigue resistance to creep

under temperature fluctuation

is more than three times

better for forging

The formula for calculating surfacestress during frequent temperaturefluctuations is:

( ) ( )1

S N m fE

S K K T T

=

For a thermal shock of 100F,average values for F22 andF91 at Tm = 400F are:S = Surface stress psi

E = 28.8 106 psi (modulus ofelasticity)

= 7.65 10-6 in./in.F(coefficient of thermalexpansion)

Tm Tf= 100F (metal temperaturebefore shock minustemperature of fluidcausing shock)

= 0.3 (Poisson's ratio)Ks = Surface finish stress

intensification factor = 4.0 for castings with non-

machined waterway = 1.2 for forged machined

waterwayKn = 1 (notch stress

intensification factor,assuming no sharp edgesin stressed area)

Surface stress for forging= 37,769 psiSurface stress for casting= 125,897 psi

References:

[1] Donald R. Bush (Fisher) articlesin Valve Magazine Volume 13,Winter 2001.

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