A Computational Study of the Creep Response of High … · 2019-01-18 · 1 A Computational Study...

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A Computational Study of the Creep Response of High-Temperature Low Chrome Piping with Peaked

Longitudinal Weld Seams

ASME 2016 Pressure Vessels and Piping Conference July 17-21, 2016

Vancouver, British Columbia, Canada

Phillip E. Prueter, P.E.Jonathan D. Dobis, P.E.

The Equity Engineering Group, Inc.

Mark S. GeisenhoffMichael S. CayardFlint Hills Resources

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2 © 2016 E2G | The Equity Engineering Group, Inc. All rights reserved

INTRODUCTION• There have been numerous failures of high-temperature, low chrome piping in the

power generation and petrochemical industries• Several of these failures have been attributed to peaking of longitudinal weld seams. • Generally, local weld peaking occurs during pipe manufacturing due to angular

misalignment of the rolled plate at the weld seam location • Many fusion-welded piping fabrication standards have no specific tolerance for

longitudinal weld seam peaking; some of the high-temperature pipes that have failed in-service met the required original fabrication tolerances.

• Depending on original heat treatment, creep damage progression is known to be accelerated by the mismatch in creep properties between the base metal, weld deposit, and heat affected zone (HAZ).

• Local weld seam peaking can induce significant local bending stresses in the pressure boundary, and the presence of local peaking can lead to creep crack initiation and propagation, and eventual rupture of the pressure boundary.

• An overview of some of the well-known historical low chrome piping failures is provided in this study.

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ANALYSIS OVERVIEW• Finite element analysis (FEA) is employed and coupled with non-linear simulation

techniques to investigate the creep response of piping with peaked long-seams. • The objective is to estimate the remaining life of low chrome piping geometries

and to assess the sensitivity in results to variations in key parameters such as operating temperature, magnitude of peaking, and the effect of heat treatment.

• Commentary on different creep damage failure criteria is rendered, and the effect of implementing a damage parameter that adjusts the elastic modulus of the material as a function of creep damage accumulation is examined.

• The simulations utilize the Materials Properties Council (MPC) Omega creep methodology and compare the creep damage progression for cross-sections of 30 and 36-inch diameter 1 1/4 Cr - 1/2 Mo pipes with and without peaking.

• These simulation techniques are valuable in estimating remaining life of in-service piping, and can be leveraged to establish recommended local peaking fabrication tolerances, appropriate inspection practices, and reasonable non-destructive examination (NDE) intervals for in-service, high-temperature low chrome piping systems.

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BACKGROUND INFORMATION• Early high-temperature failures of 1 1/4 Cr - 1/2 Mo components, such as super-

heater outlet headers and piping components were attributed to cracking. • In 1968, the ASME Code reduced the (time-dependent) allowable stresses for 1

1/4 Cr - 1/2 Mo materials. • The allowable stresses at 1,000°F and 1,050°F were reduced by 16 and 26

percent, respectively. • Headers and piping components operating in the creep regime designed

during the 1950s and 1960s are potentially under-designed. • A second decrease in allowable stresses took place in the 1989 addenda to the

ASME Code, where the allowable stresses for 1 1/4 Cr - 1/2 Mo decreased again.• ASME B31.3 introduced a weld joint strength reduction factor (W) in the 2004

Edition. The intent of this factor is to account for long-term behavior of welds at elevated temperatures in the absence of creep tests (above 950°F).

• ASME B31.1 introduced this parameter in the 2008 Edition.

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OVERVIEW OF INDUSTRY FAILURES• Major contributors to industry failures include local stress increase from long-

seam peaking, and the effects of creep strength (rate) mismatch of base metal, weld HAZ, and/or weld deposits at the welded joint.

• Long-seam failures have been observed in chrome-moly welds given sub-critical PWHT as well as those given solution anneal followed by tempering.

• The Mohave steam pipe failure (1-1/4Cr alloy) in 1985 and Monroe (2-1/4Cr alloy) in 1986 showed direct evidence of a progression of cracking from sub-surface originations in regions of high stress multiaxility due to local creep property mismatch and joint geometry.

• The cracking was preceded by extensive cavitation in the weld HAZ or at the weld fusion line facilitated by the local triaxial stress state associated with the creep rate property mismatches in the weld zone.

• The effects of the different material property zones at the weldment lead to stress intensification and triaxial tension that accelerates the rate of cavity growth near the weldment and occurs in or is adjacent to the HAZ. These systems operated at roughly 1000°F.

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OVERVIEW OF INDUSTRY FAILURES (CONTINUED)

• It has been confirmed in postmortem investigations (documented in WRC Bulletin 475) that the creep damage initiated subsurface due to high triaxial tension.– The creep damage progressed to an advanced state without any apparent OD

surface evidence. • In a 1995 failure investigation of a refinery long-seam piping failure (catalytic

reformer unit), the metallurgical investigation and creep testing indicated that the 1-1/4 Cr-1/2 Mo pipe material exhibited greater than average creep strength and creep ductility.

• Nevertheless, the pipe failed after only 100,000 hours at a nominal hoop stress of 6 ksi with an operating temperature range of 970°F to 1000°F.

• Results from subsequent detailed FEA indicated very high bending stresses were present in the pipe due to peaking at the long-seam weld, – This accelerated the damage at the HAZ fusion line until an 18-inch long

through-wall crack developed. – Peaking on the order of 1/8 inch can result in a stress increase of about 2.5

times the nominal hoop stress from pressure.

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LONGITUDINAL SEAM PEAKING

• Peaking induces a bending stress local to the peaked seam, with high stress on the OD of the vessel in the case of inward peaking and high stress on the ID in the case of outward peaking.

• Part 8 of API 579 provides calculation methods for determining the additional elastic bending stress due to local peaking.

• This closed-form method is typically used to evaluate the vessel long seams (pressure loading only) for failure from fatigue (for cyclic equipment).

• The level of weld peaking in vessel or piping long-seam welds is a critical factor when considering both fatigue and creep response.

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PEAKING MEASUREMENTS• For cylindrical or spherical shells, the angular

misalignment at a given joint can be established by using a centered or rocked template.

• Understanding the margin of error associated with peaking measurements is critical in establishing the risk for premature failure.

• Using templates introduces human error into the inspection process. As a result, some owner-users are opting to use more advanced inspection techniques such as laser scanning technology to attempt to quantify peaking magnitudes.

• Still, for many in-service piping systems, it is often not practical nor is it feasible to acquire peaking measurements for the entire system.

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STRESSES FROM PEAKING• The plot below shows the effect of peaking on bending stresses for the 30 and

36-inch piping at typical operating pressures.

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MATERIAL PROPERTIES• A691 Class 22 material is stress relieved but not normalized and tempered,

and welds are two-sided, full-penetration welds.• This is an important distinction when determining creep properties for the

weld region.• Tolerances for local long-seam peaking are not explicitly included in the

ASTM material specification.• Given that the pipe material was not normalized and tempered after welding,

the following Omega properties are employed.– For the weld deposit, standard 1.25 Cr Normalized and Tempered

properties (from API 579) are employed. – Standard annealed 1.25 Cr properties (from API 579) are used for the base

metal. • HAZ creep properties are adjusted to achieve debited creep properties:

– The creep rate is increased by roughly a factor of 4.– Omega is decreased by roughly 25%.

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FINITE ELEMENT ANALYSIS• An idealized, 2D generalized plane strain, half-symmetry model is constructed.• Material property definitions are shown.

HAZ (Custom 1 1/4 Creep Properties)Base Metal (1 1/4 Cr Annealed)Weld Deposit (1 1/4 Cr Normalized and Tempered)

0.05”

0.10”

30°

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EFFECT OF LOCAL PEAKING• Creep damage for varying levels of local

peaking for the 36-inch pipe at design conditions (worst-case operation for 500,000 hours or roughly 57 years) is plotted.

• The effect of peaking on creep damage is evident; even 1/16 of an inch of local peaking causes a significant portion of the HAZ to reach 100 percent damage after 500,000 hours.

• The 1/4 inch peaking simulation failed to converge due to excessive damage after roughly 350,000 hours.

• Even the model with no peaking reaches 100 percent damage locally in the HAZ after 500,000 hours.

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EFFECT OF OPERATING TEMPERATURE

• To evaluate the sensitivity to operating temperature, several cases are considered for the 1/8 inch peaked pipe at design internal pressure: 1,000°F, 975°F, 950°F, and 925°F.

• Creep damage accumulation in the 36-inch pipe after 500,000 hours for these different constant operating temperatures is plotted.

• These contour plots demonstrate the significant sensitivity to operating temperature.

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EFFECT OF HEAT TREATMENT• A simulation with uniform normalized and tempered Omega properties is

compared to the model with adjusted/custom properties.• Normalizing and tempering after welding is assumed to remove the property

mismatch between the HAZ and the surrounding base metal and weld deposit. • There is a significant improvement in creep damage for the normalized and

tempered model. The local damage accumulation on the ID near the weld centerline (symmetry plane) is likely an artifact of the idealized weld geometry.

Stress Relieved (PWHT)Normalized and Tempered

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DEFINITION OF FAILURE• Applying the results of numerical simulations to predicting creep failure is challenging.• The difficulty is in defining what truly constitutes “failure.”

– Damage accumulation reaching 100% at a point?– Damage accumulation reaching 100% over a finite length?– Through-wall damage accumulation above a given threshold?

• For many of the industry steam line failures, a progression of cracking from sub-surface originations in regions of high stress multiaxility were cited as the cause of failure.

• This phenomenon is difficult to fully quantify simply using FEA techniques; numerical predictions are by no means absolute. There are unknowns that must be considered.– Weld geometry (weld cap contour, penetration), lack of fusion following fabrication.– Weld filler metal, variations in welding techniques, and original heat treatment.

• A point in the HAZ reaching 100% damage, or close to it, may indicate susceptibility to creep crack initiation, but not necessarily imminent failure.

• Inspection can be used as a tool to provide insight and to better understand the risk associated with creep crack growth that ultimately leads to failure.

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ELASTIC DAMAGE PARAMETER• To account for changes in the elastic modulus as creep damage accumulates, an

elastic damage parameter is implemented in the FEA following:

• A comparison with no elastic damage and an elastic damage threshold set to 10% is shown.

• As creep damage surpasses 10%, the elastic modulus begins to linearly decrease to zero between 10% and 100% damage.

• While using a damage threshold of 10% may not be truly representative of the welded region, this example highlights how implementing an elastic damage parameter can change the overall damage progression through the HAZ at a welded joint.

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DAMAGE EVOLUTION• Creep damage evolution at varying times with

an elastic damage threshold of 10% is shown. • This simulation reaches failure at roughly

225,000 hours; this is attributed to the creep damage that has progressed through the entire thickness of the HAZ and reached a magnitude of 100%.

• Creep damage progression along the weld fusion line between 150,000 hours and 225,000 hours is evident.

• The implementation of an elastic damage parameter is required to achieve this behavior in the FEA, which is fairly indicative of how a crack would progress through the HAZ or along the fusion line (consistent with documented industry failures).

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EFFECT OF ELASTIC DAMAGE THRESHOLD• Simulations with a higher elastic damage threshold (10% vs. 80% shown below)

predicts damage progression further through the wall thickness in the HAZ (along the boundary between the HAZ and the adjacent weld deposit/base metal).

• This is an artifact of the primary stresses not being redistributed over a wider portion of the HAZ (as is the case when the elastic damage threshold is 0.1).

• This is more indicative of failure from propagation of crack-like flaws because highly localized damaged regions are surrounded by relatively stiff adjacent material.

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SUMMARY AND CONCLUSIONS• The creep response of high-temperature 1 1/4 Cr - 1/2 Mo piping with

longitudinal weld seams is very sensitive to the magnitude of local peaking as well as historical operating pressures and temperatures.

• Reasonably predicting the remaining life of these piping systems is of great interest to owner-users, as there have been numerous catastrophic failures of low chrome long-seam welded piping in the refining (normally, in catalytic reforming units) and electric utility industries.

• For new piping designs, seamless piping is clearly preferred; however, if long-seams are inevitable, minimizing the creep property mismatch between the weld deposit, HAZ, and adjacent base metal through normalizing and tempering after welding is an effective way to improve creep behavior. – For piping made to the A691 specification, this would be achieved by

purchasing pipe fabricated to Class 40, 41, 42, or 43. – Additionally, strict tolerances for the magnitude of permissible local

peaking should be prescribed prior to fabrication.

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SUMMARY AND CONCLUSIONS (CONT.)• For existing, high-temperature piping systems, rigorous inspection, for both

peaking and cracking, can be cumbersome and often times not feasible due to accessibility and the size of the piping systems.

• Focusing inspection on sections of piping that have historically operated under the most severe pressure and temperature combinations is one way of attempting to manage the risk associated with potential creep failures.

• It is important to quantify the magnitude of long-seam peaking in existing piping systems, to understand historical operating temperature and pressure trends, and to accurately document future operating conditions.

• Numerical creep simulations and parametric studies can be coupled with inspection to estimate the risk associated with operating in-service piping. – An elastic damage parameter can affect the overall creep damage

progression through the HAZ and ultimately, influence remaining life predictions and the creep damage profile.

– Establishing appropriate elastic damage parameters for a given material or welded geometry can be supplemented with material creep testing.

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Corporate Headquarters20600 Chagrin Boulevard, Suite 1200Shaker Heights, OH 44122

Satellite OfficesHouston, TXVictoria, TXAlberta, Canada

216.283.9519www.EquityEng.com

Corporate Headquarters20600 Chagrin Boulevard, Suite 1200Shaker Heights, OH 44122

Satellite OfficesHouston, TXVictoria, TXAlberta, Canada

216.283.9519www.EquityEng.com

Phillip E. Prueter, P.E.Principal Engineer

The Equity Engineering Group, Inc.Shaker Heights, Ohio [email protected]

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A Computational Study of the Creep Response of High … · 2019-01-18 · 1 A Computational Study...

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