lecture3_1

IIW-AWS

Technical Lectures

The Cr-Mo Steels

January/February 2006

J. F. Henry

Temper Embrittlement

Lesson 3IIW-AWS


Temper EmbrittlementTemper Embrittlement

Lesson 3IIW-AWS


• Temper Embrittlement is defined as the shift in the ductile-brittle transition temperature (DBTT) that results when certain alloy steels are held within, or cooled slowly through, a critical temperature range (~700-1000 °F).

• An IIW document summarized that temper embrittlement affects the toughness of Cr-Mo steels that are exposed to temperatures 660-1110 °F.

• Slow cooling through the critical range following tempering or PWHT, or service exposure in that temperature range, can lead to embrittlement.

Temper Embrittlement DefinedTemper Embrittlement Defined

Lesson 3IIW-AWS


• Temper embrittlement is a major cause of degradation of toughness of ferritic steels.

• Numerous otherwise sound components must be retired due to severe embrittlement during elevated temperature service, since under these conditions the critical crack size can become very small.

Effects of Temper EmbrittlementEffects of Temper Embrittlement

Lesson 3IIW-AWS


Effects of Temper EmbrittlementEffects of Temper Embrittlement

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• A significant number of components are designed to operate in the susceptible temperature range, so that temper embrittlement can occur during service. This group involve components of all section sizes, including boiler headers, steam pipes, turbine casings, pressure vessels, blades, fasteners, HP-IP rotors, & turbine disks.

• In the case of massive components, such as LP rotors, generator rotors, and retaining rings, some embrittlement may be inevitable as a result of slow cooling following heat treatment.

Service and Heat TreatmentService and Heat Treatment

Lesson 3IIW-AWS


• Temper embrittlement has been identified in a wide range of alloys, including low alloy steels, higher strength alloy steels and stainless steels.

• Risk is higher with components produced using “older” methodologies. Increased susceptibility is related to:

– Higher normalizing temperature larger grain size– Steel making practices higher levels of impurities

Material AffectedMaterial Affected

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• In general, the increased risk of rapid brittle fracture is not a major concern during steady operation at elevated temperature --- sufficient fracture toughness.

• Problems have been encountered during hydrostatic testing of headers & piping or during operational transients, where fracture toughness is critical.

• The risk of failure should also be considered during steady operation in the case that stable crack growth can lead to instability during subsequent operational transient.

• Reduction in upper shelf energy.

Operational RisksOperational Risks

Lesson 3IIW-AWS


Decrease Of Critical Flaw Size For Brittle Fracture Of A 2 ¼Cr-1Mo Decrease Of Critical Flaw Size For Brittle Fracture Of A 2 ¼Cr-1Mo Reactor Vessel At 50 F Due To Temper EmbrittlementReactor Vessel At 50 F Due To Temper Embrittlement

Operational RisksOperational Risks

Lesson 3IIW-AWS


• Occurs within a specific temperature range.

• Related to compositional changes in grain boundaries.

• Intergranular fracture.

• Increase in FATT or in DBTT, but strength & hardness are largely unaffected.

• Can be avoided or eliminated by heat-treating above the susceptible temperature range followed by rapid cooling through that range.

Characteristics Of Temper Characteristics Of Temper EmbrittlementEmbrittlement

Lesson 3IIW-AWS


Intergranular Fracture Produced Intergranular Fracture Produced By Temper Embrittlement By Temper Embrittlement In A Cr-Mo SteelIn A Cr-Mo Steel

Fracture Behavior in Embrittled Fracture Behavior in Embrittled MaterialMaterial

Lesson 3IIW-AWS


Shift In Transition Curve Shift In Transition Curve Due To Temper EmbrittlementDue To Temper Embrittlement

Lesson 3IIW-AWS


• Temper embrittlement occurs by equilibrium grain boundary (GB) segregation of major alloying elements and impurities occurring in the susceptible temperature range.

• Impurities reduce the cohesive strength of grain boundaries, creating an easy path for fracture.

• When the GB energy of a material is reduced by the presence of of an alloy element, the concentration of the element in the GB is higher than in the matrix due to the “disordered” state of the GB compared to the matrix.

• Segregation effect is usually confined to one to two atomic layers and decays exponentially away from GB.

• Segregation is reversible at temperatures above the susceptible temperature range

Mechanisms Related To Temper Mechanisms Related To Temper EmbrittlementEmbrittlement

Lesson 3IIW-AWS


• In GB segregation theory, GB solute concentration Xb is approximately given by Langnuir-Mclean equation:

Where is the free energy released per mole when a solute atom is released from the matrix to the GB. is usually positive and increases as the size difference between the solute and the matrix atoms increases.

)exp(0 RTGXX b

b

bGbG


Lesson 3IIW-AWS


Auger Spectra From A Ni-Cr Steel In The (a) Embrittled And (b) Non-Embrittled Conditions, Showing Segregation Of Phosphorus Onto Grain Boundary

Embrittled Non-Embrittled

Segregation Effect During Segregation Effect During EmbrittlementEmbrittlement

Lesson 3IIW-AWS


Dependence Of GB Concentration Of Phosphorus On Annealing TemperatureFor Fe-P Alloys With Different Levels of Phosphorus

• GB segregation of P decreases with increasing temperature and decreasing bulk concentration

Temperature/Concentration Temperature/Concentration DependenceDependence

Lesson 3IIW-AWS


GB Concentration Of P and C In Fe-0.17%P Alloys With Different Carbon Levels

• GB concentration of P decreases with increasing free carbon content

• The Plateau shows at a carbon level of 55 ppm (solubility limit at 660 °C)

Carbon-Phosphorus InteractionCarbon-Phosphorus Interaction

Lesson 3IIW-AWS


Effects Of C and Cr On GB Concentration Of P After Annealing At Different Temperatures With About The Same Bulk Concentration of P

• The level of P segregation dramatically decreases when carbon is added to Fe-P

• With the addition of 2%Cr, the segregation of P is nearly the same level as in the alloys without carbon

C-Cr-P InteractionC-Cr-P Interaction

Lesson 3IIW-AWS


• In Seah-Hondros segregation theory, GB enrichment ratio is inversely proportional to atomic solubility of the element in the parent lattice.

• The solubility in turn is inversely proportional to the term ( ) in the Gibbs absorption formula:

Where is the GB concentration of the impurity in excess of the bulk concentration C. is reduction in GB energy with the the concentration of the impurity at temperature T.


dCd

dCd

kTC 2

2

2

dCd

Lesson 3IIW-AWS


Correlation Of Predicted GB Enrichment Ratios For Various Solutes With The Inverse Of Solid Solubility

Effect of Matrix Solubility of Potential Effect of Matrix Solubility of Potential Embrittling AgentsEmbrittling Agents

Lesson 3IIW-AWS


• Time-Temperature relationship for temper embrittlement follows “C-Curve” behavior---Diffusion-Controlled Process.

C-Curve Behavior For A 2 ¼ Cr-1 Mo Steel Showing Isothermal FATT Contours

Embrittlement KineticsEmbrittlement Kinetics

Lesson 3IIW-AWS


• No significant segregation of impurity elements had been found in carbon steels, presumably due to sufficient free C in solution. Embrittlement can occur when carbon is tied up as carbides.

• Embrittlement occurs in commercial alloy steels due to segregation of P during slow cooling from high temperatures involved with normalizing, during controlled step-cooling and on holding in the temperature range (650-1000 F)

Factors Affecting Temper Embrittlement

Lesson 3IIW-AWS


Increase In Brittle Behavior, As Measured By Increasing Values Of FATT With Increasing Levels Of Bulk P Content In Three Rotor Steels

Effects of Alloy ContentEffects of Alloy Content

Lesson 3IIW-AWS


• Sn, Sb & As can also lead to embrittlement in some special alloys, especially when P is absent. When P is present, it appears to exhibit the dominant effect. Presence of C can further lower the tendency to embrittle.

GB segregation of Sn in Fe-0.2%Sn Alloy

GB segregation of Sn in Fe-Sn-C Alloys


Lesson 3IIW-AWS


Embrittlement Data For A Range Of 2CrMo Weld Metals

• Mn & Si in combination appear to affect the level of embrittlement when P is present.

– Mn is believed to reduce the GB strength– Si is believed to promote the segregation of P– A systematic effect of greater embrittlement due to P at higher levels of the sum of Mn & Si

• The presence of Mo Acts to slow down the embrittlement due to P

– either because Mo & P atoms tend to associate and prevent P segregation– or because Mo increases the coherency of GB Structure


Lesson 3IIW-AWS


• The rate of embrittlement with time follows a parabolic relationship (diffusion-controlled process).

Variation of FATT With Time Of Aging at 850 °F For A CrMoV

Rotor Steel

• Segregation effects are reversible and embrittlement can be removed by higher-temperature heat-treatment followed by rapid cooling.


Lesson 3IIW-AWS


• Temperature of exposure is an important factor for embrittlement --- “C-Curve Behavior”

• A steep segregation gradient in GB area -- peak segregation occurs within 2-3 atomic planes, indicating that energy considerations limit segregation to within the peak regions of GB structure

• There appears to be a trend in decreasing susceptibility to embrittlement from martensitic to bainitic to ferritic structure.

AES results showing S, P & Sn GB segregation pattern


Lesson 3IIW-AWS


• Grain size appears to be an important factor, since available GB area is governed by grain size.

Variation of FATT With Prior Austenite Grain Size At Fixed Hardness and Impurity Levels


Lesson 3IIW-AWS


Compositional Effects on EmbrittlementCompositional Effects on Embrittlement

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Significant Improvement In The Levels Of Trace Element In Modern Steels

Reduction Of The Level Of Trace Elements With Time For 2 ¼ Cr-1 Mo Steel Components

Impact of Steel-Making PracticeImpact of Steel-Making Practice

Lesson 3IIW-AWS


• Since temper embrittlement is mainly related to material chemistry, susceptibility can be quantitatively evaluated in terms of composition.

• J Factor

J = (Mn+Si)(P+Sn) x 104 (%)FATT (°C) = 0.38 J –45

• X Factor

= (10P+5Sb+4Sn+As)/102 (ppm)X’ = (8P+10Sb+4Sn+As)/102 (%)

Prediction Of Temper Embrittlement

X___

Lesson 3IIW-AWS


• PE Factor

PE = C+Mn+Cr/3+Si/4+3.5 (10P+5Sb+4Sn+As)

• FATT Factors

FATT (°C)= 7524P+7194Sn+1166As-52Mo-450,000(P x Sn) (%)

FATT (°C)= 4.8P+24.5Sn+13.75 (7-GS) +2 (Rc-20)+0.33 (Rc-20)(P+Sn) + 0.036 (7-GS)(Rc-20)(P+Sn) (%)

Where: GS—Grain Size In Form Of ASTM Number, Rc---Hardness (HRC)


Lesson 3IIW-AWS


FATT (°C)= 4.8P+24.5Sn+13.75 (7-GS)+2 (Rc-20)+0.33 (Rc-20)(P+Sn)

+ 0.036 (7-GS)(Rc-20)(P+Sn) (%)

Where: GS—Grain Size In Form Of ASTM Number, Rc---Hardness (HRC)


Lesson 3IIW-AWS


• EF Factor

EF = %Si + %Mn + %Cu + %Ni x Y Y = (10P+5Sb+4Sn+As)/102 (ppm)

• 50%FATT Factor (C-Mn Steels)

FATT (°C) = 19 + 44 (%Si) + 700 (%free nitrogen)1/2 + 2.2 (%pearlite) + 11.5 d–1/2

Where: d—linear ferrite grain size (mm)

* The influence of elements on FATT was also assessed in an overall ranking of 1% Si: FATT + 44 °C 0.01% N: FATT + 70 °C 1%Sn: FATT + 136 °C 1%P: FATT + 459 °C


Lesson 3IIW-AWS


• Step Cooling An Accelerated Embrittlement Process A commonly used procedure for the assessment of

pressure vessel steels in the petroleum industry According to API results, FATT in such a step-

cooling treatment is approximately one third of what might be expected after 30 years of service in an actual reactor vessel.

• Long-Term Isothermal Aging

Evaluation Of Susceptibility To Temper Embrittlement (Laboratory Methodology)

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