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Welding of ferritic creep-resistant steels

Job Knowledge

Creep is a long term failure mechanism that, in most metals, occurs at elevated temperatures (see Job Knowledge No. 81 ). Creep strength in the ferritic steels is achieved by alloying with elements that will provide enhanced strength at high temperatures. Chromium (Cr) and molybdenum (Mo) are the two principal alloying elements but vanadium (V) and niobium (Nb) may also be added.

Table 1 gives the nominal composition of the commoner creep resistant steels. In addition to the use of these steels in creep service they also have resistance to hydrogen attack and corrosion by sulphur bearing hydrocarbons. They are therefore found in power generation and the oil and gas industries.

Table 1 Nominal composition and mechanical properties of the creep resistant steels

Steel grade Composition - nominal % Mechanical properties - typical
  C max Cr Mo V Nb UTS
N/mm2
0.2% Proof
N/mm2
Elongn
%
Charpy-V
J/°C min
C1/2Mo 0.3   0.5     520 320 35 27@20
½/½/¼CrMoV 0.14 0.5 0.6 0.25   585 330 25  
1¼Cr½Mo 0.13 1.2 0.5     530 350 35 27@20
2¼Cr1Mo 0.13 2.25 1.0     555 350 35 27@20
5Cr½Mo 0.13 5.0 0.5     690 475 28 27@23
9Cr1Mo 0.13 9.0 1.0     675 475 30  
9CrMoVNb
(9Crmod or P91)
0.13 8.75 1.0 0.23 0.08 650 480 30 40@20

The creep resistant steels all contain strong carbide and/or nitride forming elements. These are intended to provide a fine dispersion of precipitates that both increase the tensile strength and impede the formation of the voids illustrated in Fig 1 and Fig 2 of Job Knowledge No. 81. Chromium is also added to reduce the scaling or oxidation of the steel at high temperatures. Each steel grade has a creep limit (a stress and temperature above which it should not be used) and a similar limit on oxidation resistance. The allowable temperature increases with the alloy content, enabling the more highly alloyed steels to be used up 650°C.

The ½/½/¼CrMoVsteel is a special case. It was developed for the power generation industry in the UK and is unlikely to be encountered elsewhere but some notes have been included as it may be found in older plant scheduled for repair.

As the alloy content increases then so does the hardenability (the ability to form martensite) of the steel. C½Mo, CMV and 1¼Cr½Mo steels form ferritic/bainitic structures, the other more highly alloyed steels forming martensite, even at relatively slow cooling rates. This should give some hint as to one of the problems encountered when welding this family of steels; that of hydrogen induced cold cracking (see Job Knowledge No. 45), since martensite is generally hard, brittle and sensitive to the presence of hydrogen. Low hydrogen welding processes are therefore essential. This includes ensuring that any shield gases are of high purity and are dry; ideally with a dew point less than 50°C.

Preheat is essential for most of the alloys (the IIW carbon equivalent method is not valid for these grades of steel) and few welding specifications give much guidance regarding recommended preheat temperatures. However, ASME B31.3 and EN 1011 Part 2 both contain recommendations. Table 2 is adapted from the EN specification for processes with hydrogen limited to between five and 10mls of hydrogen in 100gms of weld metal (Scale C). It may be permissible to use lower preheats if the hydrogen content is reduced to less than 5mls/100gm; for instance when depositing a TIG root pass. This could be confirmed during welding procedure development.

Table 2 Recommended preheat and interpass temperatures

Steel Grade Thickness
(mm)
Min. Preheat
(°C)
Max. Interpass
(°C)
C1/2Mo ≤15
>15≤30
>30
20
75
100
250
½/½/¼CrMoV All 150 300
1¼Cr½Mo ≤15
>15
100
150
300
2¼Cr1Mo ≤15
>15
150
200
350
5Cr½Mo All 200 350
9Cr1Mo All 200 350
9Cr1MoVNb All 200 350

11/4Cr1/2Mo power station boiler header
11/4Cr1/2Mo power station boiler header

An additional problem that may be encountered with the creep resistant steels is that of reheat cracking (see Job Knowledge No. 48 ). This is a cracking mechanism that takes place, as the name suggests, during reheating of the welded joint, either when the weld is post weld heat treated (PWHT) or is put into high temperature service without PWHT.

The most sensitive grades are those containing vanadium; the ½/½/¼CrMoV steel being one of the most sensitive. It is so sensitive that it may be necessary to maintain the preheat and hot grind and blend the weld toes of a thick, highly restrained weld to reduce stress concentrations before immediately performing the PWHT operation.

Solutions to this problem are control of residual elements to low levels, low heat input to minimise grain growth in the HAZ and devising a welding procedure that results in the maximum amount of grain refinement in the HAZ. Rapid heating through the temperature range 350 - 600°C at which the steel is most sensitive can also help. This approach must be treated with some caution as too rapid a temperature rise can cause unacceptable stresses and distortion and may violate code requirements.

Most of the creep resistant steels require PWHT; mandatory in all of the application codes. This is to ensure that the hard microstructures formed during welding are softened and toughness improved. It is also necessary to heat treat the weld and HAZs to ensure that the precipitates, required to give best creep performance, are of the correct size and distribution. PWHT temperatures and soak times must therefore be closely controlled to develop the required mechanical properties. Typical temperatures and times are given in Table 3. These figures are typical only and it is important that the item is heat treated precisely in accordance with the relevant application code; ASME VIII, BS PD5500, EN 13445 etc.

Table 3 Typical PWHT temperatures and times

 

Steel Grade Temperature
Range (°C)
Soak Time
(hours)
C1/2Mo 630 - 670 1 per 25mm
½/½/¼CrMoV 650 - 680 1 per 25mm
1¼Cr½Mo 650 -700 1 per 25mm
2¼Cr1Mo 680 - 720 2 min
5Cr½Mo 710 - 750 2 min
9Cr1Mo 730 - 760 2 min
9Cr1MoVNb 730 - 760 2 min

The PWHT temperature of the 1¼Cr½Mo and the 2¼Cr1Mo steels are sometimes changed from the ranges given in Table 3 in order to develop specific properties; see for example Table 4.4.1 in BS PD5500.

The 9CrMoVNb steel is particularly sensitive to PWHT times and temperatures and great care must be exercised when PWHT'ing this particular grade of steel.

Any alloy containing more than 2% chromium will need to be bore purged with an inert gas such as argon when depositing a TIG root pass. Exposure of the molten weld pool to the atmosphere results in some of the chromium boiling off giving rise to a porous or 'coked' bead on the reverse side of the weld. This adversely affects both mechanical properties and corrosion resistance.

Welding consumables matching the parent metal composition are readily available for all of these steels for most of the welding processes. An exception to this is the ½/½/¼CrMoV steel which is conventionally welded with a 2¼Cr1Mo filler. Dissimilar metal joints made between components from within this group of ferritic steels or with the carbon manganese steels are usually welded using a filler metal that matches the less highly alloyed steel. PWHT temperature for the dissimilar metal joints can be a problem and tends to be a compromise between overtempering the lower alloyed steel and undertempering the more highly alloyed metal.

This article was written by Gene Mathers.