use of replication and portable hardness testing for high temperature plant integrity and life...

Seminar Power & Process Plant Issues

Lahore, Pakistan: May 2011

Use of Replication and Portable Hardness Testing for High

Temperature Plant Integrity and Life Assessment

M Hussain, ETD Pakistan

[email protected], +92 345 812 4575 1

mailto:[email protected]

European Technology Development Ltd.

Overview

Material degradation assessment is one of the most important fields of life assessment

It allows to identify, localize and quantify the damage mechanism impact in a certain component

It is used worldwide specially in the power plants (conventional and CCGTs), petrochemical and process industries

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Introduction

Life assessment based on calculational procedures is conservative (use minimum parent material properties, etc)

Difficult to account for welds

Metallographic methods assess the actual material condition of the component/weld

Metallographic replication

Hardness assessment


Life Limiting Areas

Welds are frequently the locations for high temperature plant failures

HAZ with differing properties

Residual stresses

Fabrication defects

Welds are often located at stress concentrations

Welds need detailed evaluation of defects and properties

NDE and possibly materials testing carried out


Microstructure & Damage

Microstructure degrades & creep cavitationdamage develops as the consumed life fraction increases

Can see these changes if polish and etch at site

Hard to use microscope in power plant

Need flat surface - Not normal in plant

Hence replication


Creep Damage Accumulation

Isolated and oriented cavities, linking to form micro-cracks, then macro-cracks


Precipitate Coarsening

Progressive coarsening of precipitates in a low alloy steel


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Metallographic Replication

Must be targeted at the areas most likely to show creep damage

Normally welds and bends in pipework

High temperatures; end-loads on pipework; known problems

Apply across Weld, HAZ and Parent material

Examine replicas at site optically before re-lagging

Further examination in laboratory


Replication Procedure

1. Area is grinded to high metallographic standard

2. Then polished and etched several times to remove all traces of cold work from polishing

3. Finally surface wetted with acetone and acetate foil applied

4. Acetone softens foil and capillary action forces the foil to conform to the etched structure

5. After drying, the foil is removed with image of the etched surface impressed on foil surface



Film

Film

Film

1) Rough component surface

2) Component surface polished

3) Component etched to reveal microstructure

(Steps 2 and 3 repeated ~4 times)

4) Film applied to surface

5) Film conforms to surface as solvent dries

6) Film stripped from surface with record of

microstructure



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Polishing of a P91 reducer welded to a CrMoV HPHT

steam turbine valve

Replication of etched cross weld surface

(the arrows show the acetate foil)


Replica Positions Pipework

4 points round welds

Intrados, extrados of bends

Intersections - saddles and crotch positions

Tubing Hottest tubes - superheater, reheater

outlets

Swollen tubes to quantify remaining life

Headers Antler/stub tubes – minimum grinding

Plain barrel if any signs of distress

Nozzles

Turbines Rotor bores


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Portable grinding and polishing equipment

Grinding and polishing head

Control unit


Sentencing of Replicas –Quantifying the Damage

Several means of quantifying damage: ‘A’ parameter depends on

counting the number of cavitated grain boundaries Gives numerical answer but

time-consuming

Normal use life based on damage classification schemes

Action may be advised but depends on History

Future outage schedules

Operating practice

Best assessed individually


Cavitation Damage Classification

A: Clear

B: Isolated cavitation

C: Orientated cavitation

D: Microcracking <2mm

NDE non-detectable

E: Macrocrack >2mm

NDE detectable

After Neubauer


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Introduction to Hardness

Hardness is used in power, petrochemical and process industry for:

quality control

life assessment

It is defined as the ability of a material to resist permanent indentation or deformation when in contact with an indenter under load

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Introduction to Hardness

Basically a hardness test consists of pressing an indenter of known geometry and mechanical properties into the test material

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Hardness – Life relationship

Hardness indentation technique is one of the oldest testing methods applied to analyse the materials properties

It gives the hardenability of a certain component

Conversion charts can convert hardness values to tensile strain and consequent probability to type IV damage.

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Hardness changes and testing

Several investigators have developed hardness models to calculate the remaining life of piping and tubing components.

Models based on hardness have been developed for low alloy steels and modern steels based on creep data.

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Portable hardness testing equipment

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MIC 10 control unit

Vickers 5kgf Probe


Hardness test for superheater header

Shape of indent of on-site hardness measurement

Robertson D. et al; ETD Lifing Procedure

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Main steam line

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Material Properties

Material of pipe A 335 P11

Material of fittings A 234 WP11

Pipe Geometry

Outer diameter, mm 450

Wall thickness, mm 40

Operating Conditions

Pressure, Bar 95

Temperature, C 530

Service time, hours* 180,071Case Study from Recent ETD Work in Europe


Main steam line (contd.)

Base Metal microstructures consisted of ferrite and bainite (or ferrite and pearlite).

HAZ microstructure was bainitic and/or martensitic, the Weld Metal microstructure was martensitic.

Bainite and pearlite microstructure showed some degradation due to long-term exposure at elevated temperature (i.e. spheroidisation)

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Pipe thermally degraded, partially spheroidized ferrite / bainite and 130HB

(400x)

Moderately degraded bainite,

partly spheroidized; 151 HB (400x)

Martensite; 166 HB (400x)

BM HAZ WM



Using optical microscopy, one of the valves, showed intergranular crack of ~2mm length in the coarse-grained region of the HAZ (CGHAZ) on the forging side of the joint (see next slide)

The morphology and the crack location were indicative of stress relief damage

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Stress relief cracking at the forging side (100X)



Hardness testing was carried out at-site for each of the replica locations.

The hardness values of Base Metal, Weld Metal and HAZ were within the expected ranges for 1¼Cr-½Mo steel after long-term, high-temperature exposure.

Some welds exhibited hardness differentials between the weld metal/HAZ and the base metal of ~60-80HB.

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Hardness (HB)

Position Weld HAZ BM

Valve Pipe

12 O’Clock 203 204 126

4 O’Clock 202 195 128

8 O’Clock 204 198 132

Avg. Hardness 203 199 129

Valve Forging Side

12 O’Clock 208 198 135

4 O’Clock 204 201 136

8 O’Clock 206 195 140

Avg. Hardness 206 198 137

The hardness test results for the base materials, HAZ and weld metals were within the expected range for P11 steel after long-term operation at elevated temperature.



The creep life fraction consumed for the components exhibiting isolated creep cavities is estimated to be 50% - as the worst case scenario. This means that the remaining creep life of these components is at least 190,000 hours.

The components examined showed limited microstructural degradation which was consistent with the plant operating conditions and service time, and the hardness levels at the examined locations were within the expected range for P11 steel after long-term operation at elevated temperature.

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Due to the presence of the micro crack, it was recommended that the valve should be re-inspected using metallographic replication and appropriate NDE (MT and UT flaw detection) after a further 10,000 hours service.

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Advantages of replication & hardness

The technique can be used easily on-site and it is non-destructive.

Good resolution of microstructural constituents if surface is well prepared. This technique has a good adaptation on flat and curved surfaces

Can be used to monitor the evolution of microstructural changes and it is useful for assessing creep, fatigue, corrosion damage in elevated-temperature components.

Can be applied to conventional materials and also to the steel alloys used in turbines and boilers.

Can be used to complement other non-destructive techniques such as ultrasonic testing.

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Limitations of replication & hardness

The technique only analyzes the microstructure of the outer surface of a material/component.

In many cases the surface microstructure can be different from the microstructure found in the interior of the component

The replica only reveals the topographic features at the surface; therefore it is impossible to analyse the chemical composition of the elements

Contamination may be a problem in harsh or dusty environments

Precision is required to operate the hardness probe

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Conclusion

It has been proved that replication and hardness can be efficiently used to perform life assessment

Sampling removal and analysis are relatively simply and not time consumable

Cost of performing these tests is lower in comparison with other non-destructive techniques

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Thank You very much for your attention.

Questions?

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use of replication and portable hardness testing for high temperature plant integrity and life...

Documents

hardness hardness

use of replication

etched surface

quantifying damage

introduction life assessment

creep cavitation damage

showcreep damage

replication procedurepolishing