Composition, Microstructure and Toughness of High Heat Steel

Paper published at the 2nd International Conference on Integrity of High Temperature Welds, 10 - 12 November 2003, Institute of Materials, London

Abstract

With the need to improve thermal efficiency, reduce costs and reduce emissions, a new generation of 9-13%Cr ferritic steels has evolved for increased service temperatures up to 620°C. There are several candidate alloys, typically containing 9-13%Cr, ~0.5%Mo and 1-2%W, with additions of Ni, Nb and V. Whilst welding consumables are available for these alloys there is a need to understand more fully the role of composition in the micro structural development and mechanical properties of weld metals. This programme of work aimed to explore the possible toughness improvement in weld metals for the W-containing steels NF616 and HCM12A through variations in the deposit chemistry. Variations in Ni and Co to the base-line weld metal compositions have been explored for both a 9%Cr and a 10%Cr series. The compositional variations studied did not significantly alter the transformed microstructure, even with Coadditions up to ~3%. All the deposits were martensitic, with, in some cases, isolated colonies of retained δ-ferrite. For both the 9%Cr and 10%Cr series, the addition of 1%Co to the base composition improved toughness, reflecting the lower hardness of these deposits. Further additions of Co did not significantly alter the base-line toughness.

Introduction

The modified 9%Cr-1%Mo, grade 91, steels are now widely used in the power generation sector for both the construction of new plant and in the upgrade and life-extension programmes for existing stations. This grade was, however, only designed for service up to 600°C, and with the more stringent requirements arising through the need to improve thermal efficiency, reduce costs and reduce emissions, a new generation of 9-13%Cr ferritic steels has evolved for service up to 620°C. By virtue of their high strength, the newer steels allow significant reductions in thickness to be made relative to conventional grades such as P91 and P22, leading to reductions in weight and to reducedfabrication costs. Extensive research and development in Japan, Europe and the USA, including a number of large collaborative projects, has led to the development of a number of candidate alloys. These alloys typically contain 9-13%Cr,~0.5%Mo and 1-2%W, with additions of Ni, Nb and V. The target properties for the parent steels included 0.2% proof and tensile strengths of 400 and 600MPa, respectively, a minimum Charpy impact toughness of 40J at 20°C, and a600°C 105h creep rupture strength of 140MPa. ^[1] Of the grades that have been developed, NF616 and HCM12A have been given ASME Code Case approval (cases 2179 and 2180 respectively).

An improvement in the mechanical properties of weld deposits of essentially matching composition should be possible, through controlled compositional variations, to support on-going steel developments for service temperatures>620°C. Work carried out by Nippon Steel on filler materials for NF616 ^[2] indicated a beneficial effect of Ni additions (up to ~0.4%) on Charpy toughness. Earlier TWI studies on weld metals for grade 91 ^[3] showed that the addition of ~1%Ni, in combination with reduced Si, Nb and N, improved toughness.

A TWI study on grade 91 weld deposits suggested that the addition of Co, like Ni, was beneficial in reducing δ-ferrite (although its effect was less potent than that of Ni), and it improved toughness. ^[4] However, unlike Ni, Co does not significantly affect the AC ₁ temperature. Tungsten has been added to the new generation of 9-13%Cr steels to improve creep resistance and high temperature strength, albeit with a small reduction in elongation. However, it is a strong ferrite former, and promotes the retention of δ-ferrite. The addition of 1%Ni or 2%Co, in combination with 1%W alloying, was sufficient to suppress the δ-ferrite retention, and improve Charpy toughness. ^[4]

Susceptibility to δ-ferrite retention is normally assessed using the chromium equivalent (Creq) and Kaltenhauser ferrite factor (FF) compositional parameters; ^[5,6] fully martensitic deposits can be achieved with Creq<8 and FF<6. ^[4] However, the FF does not include consideration of the effect of tungsten, and is thus unsuitable for the new generation steels. Morimoto et al ^[7] showed that adding 3%W to a 9%Cr steel gave ≤~25% δ-ferrite ( ≤~50% for an autogenous GTA weld). Tungsten improved high temperature strength, but markedly reduced Charpy toughness, especially after ageing [due to the precipitation of M ₂₃ C ₆ and the formation of the intermetallic Laves phase (Fe, Cr, Mo, W)]. A balance in composition is required to restrict the δ-ferrite retention and optimise the mechanical properties, particularly in weld metal, where the rapid thermal cycle promotes δ-ferrite retention. For the 9%Cr weld metals this can be achieved by, for example, a small reduction in W content relative to the base metal, and the addition of ~0.5%Ni. ^[8] For the higher Cr, HCM12A parent steel, the addition of <1.7%Cu is effective in suppressing the retention of δ-ferrite. ^[9]

This study aimed to explore the toughness improvement that could be achieved in W-containing weld deposits through variations in the deposit chemistry. Further work is, however, required to determine the effect of the compositional variations on the weld metal creep properties.

Objectives

• To determine the effect of changes in Co, Ni and Cr on microstructural development and toughness of weld metals for selected examples of the new, W-containing 9-13%Cr steels.
• To compare the toughness of the W-containing weld deposits with data previously generated for weld metals for grade 91 (W-free).

Experimental procedure

Materials

The parent steels were HCM12A produced by Sumitomo Metal Industries and NF616 produced by Nippon Steel Corporation. Their chemical compositions are given in Table 1. To provide a base-line for the weld metal studies, the current 'commercial' manual metal arc (MMA) electrodes for each material, of 4.0mm diameter, were employed. Mn, Ni and Cu levels are higher than in the corresponding parent steels, presumably to avoid the retention of delta ferrite. Experimental 4.0mm diameter MMA electrodes were produced by Metrode Products Ltd. Four different formulations were supplied for each material type, showing variations mainly in the levels of Co ( ≤3%), Ni (0 to 1%) and Cr (an extra 1% in one HCM12A deposit).

Table 1 Chemical composition of the NF616 and HCM12A parent plates, with the ASTM A335 grade P92 and ASTM A1017 grade 122 requirements provided for comparison

Ti, As, Zr < 0.01; Co ≤ 0.02; Ca < 0.001

Welding and postweld heat treatment

A 27.5° bevel with a 2mm root face was machined down one long edge of 1m long strips of the parent plate. The panel half-widths were 150mm. With a 2mm root gap, a multipass manual metal arc (MMA) weld was produced for each ofthe electrode types. The panels were restrained with strong-backs. An arc energy of 2.0kJ/mm (heat input of ~1.6kJ/mm) with a minimum preheat of 200°C and a maximum interpass temperature of 300°C were used, to allow comparison with previous studies on modified 9%Cr-1%Mo (grade 91) steel. ^[3,4]

On completion of welding, an as-welded transverse section was removed. A small sample from each weld was given a PWHT and, after metallographic examination of this section, the remainder of the panel was heat treated at 760°Cfor 2h, and furnace-cooled.

Microstructural examination, chemical analysis and hardness testing

Transverse weld sections were removed from each panel in the as-welded condition and after PWHT. The sections were prepared to a 1µm diamond finish using standard metallurgical techniques, etched in 2½ % picric acid,2½ % hydrochloric acid in alcohol, and examined by light microscopy. The compositional variations did not appear to have lowered the AC1 transformation temperature to a level below the PWHT temperature of 760°C.

The composition of each deposit was determined by direct spark optical emission spectrometry, with separate analysis for oxygen and nitrogen by inert gas fusion. Values of the chromium equivalent parameter Cr _eq ^[5] were determined for each deposit. A through-thickness Vickers hardness survey was made on each as-welded and PWHT weld section, with a 10kg indenter load, and maximum, minimum and mean values determined.

Mechanical testing

A single, all-weld-metal tensile specimen was machined from each deposit, and tested at ambient temperature to determine the 0.2% proof strength and tensile strength, elongation and reduction of area. The specimen machining and testing were to BS EN10002-1:1990. Standard 10x10mm square cross-section Charpy specimens were machined from 2mm below each of the root and cap surfaces. The specimens were through-thickness notched at the weld centreline, and tested over a range of temperature to produce transition curves. All specimen machining and testing was performed to BS EN 10045-1:1990.

Eight BxB single-edge-notch-bend, SEN(B) fracture mechanics specimens (where B is the plate thickness) were extracted from each panel, and manufactured to BS 7448:Part 2:1997. Each specimen was etched in a 2% Nital solution to assist in determining the notch location, and then notched in the through-thickness direction, at the weld centre line. The overall depth of the notch plus fatigue crack (a _o ) was approximately half of the specimen width (a _o /W ≈0.5). As the panels had been postweld heat treated, no additional treatment, e.g. local compression, was required. Each specimen was tested in three-point-bending. A double clip gauge method was used to measure the displacement of the crack faces, and the equations given in BS 7448:Part 2 were used to calculate fracture toughness in terms of crack-tip-opening-displacement(CTOD). The fracture surfaces were visually examined after testing, and key features, such as fatigue crack depth and profile and the extent of any tearing, were measured.

Results

Chemical analysis and microstructural examination

In general, the target weld metal compositions were broadly achieved, Table 2. Whilst inevitably some minor elemental variations were evident, but these are generally not considered to be significant. However, the higher oxygen levels of the experimental consumable deposits may be expected to have a slight detrimental effect on toughness.

Table 2 Chemical compositions of weld deposits

As <0.01 Pb <0.002 Sn <0.005 Ca <0.001

The microstructural examination revealed that, in all cases, the as-deposited weld metal microstructure consisted primarily of martensite. In some deposits, isolated colonies of δ-ferrite were evident. A light-etching phase was evident on some isolated grain boundaries, more noticeably in the 9%Cr deposits (W10, W12 - W15).Following PWHT, the micro-structure was predominantly tempered martensite with isolated δ-ferrite colonies, as noted for the as-welded sections. The delineation of the grain boundaries was more pronounced following PWHT, particularly in the 9%Cr deposits (W10, W12-W15).

Hardness testing and ambient temperature tensile testing

The hardness data are summarised in Table 3, and the all-weld-metal tensile test results in Table 4. The as-welded hardness and strength of each of the 9%Cr deposits and of the corresponding 10%Cr series deposits were typically similar, although the 9%Cr deposits generally had marginally greater hardness and strength than the 10%Cr deposits, reflecting the slightly higher levels of C, Mn and W. The application of PWHT for 2 hours at 760°C gave rise to a typical softening of ~130 to 150HV10. The overall range was a 98HV10 hardness reduction for the 10%Cr base-line composition to a 172HV10 reduction for the 9%Cr base-line composition. The strengths of the 9%Cr and 10%Cr deposits greatly exceeded the ASME Code Case requirements of 621 MPa minimum tensile strength and 442 and400MPa minimum yield strength for NF616 and HCM12A parent steels, respectively. The elongation and reduction of area values were low (in the range 16 to 21% and 55 to 63%, respectively).

Table 3 Summary of weld metal Vickers hardness data

Weld no/type	Vickers hardness HV10
	As-welded	PWHT
W10 (NF616 Base)
W12 (Base + 1%Co)
W13 (Base with 0.3%Ni + 1%Co)
W14 (Base - Ni + 2%Co)
W15 (Base - Ni + 3%Co)
W11 (HCM12A Base)
W16 (Base + 1%Co)
W17 (Base - Ni+2%Co)
W18 (Base + 1%Co + extra 1% Cr)
W19 (Base - Ni + 3%Co)

Table 4 Summary of all weld metal tensile test data

Toughness assessment

The Charpy transition temperature data (based on the best-fit and lower bound curves) are summarised in Table 5. In general, poorest toughness was recorded for the root subsurface location in each deposit. The two base-line deposits, W10 and W11 clearly show the poorest impact toughness. The addition of 1% Co to the base-line compositions gave the greatest improvement in impact toughness. Additions of higher levels of Co, generally improved toughness (particularly lower bound toughness), but were less effective than 1% Co.

Table 5 Summary of Charpy toughness data

* Based on computer-generated best-fit curve
** Based on visually-assessed lower bound curve

Table 6 gives transition temperature data, on the basis of the temperature to achieve a CTOD of 0.1mm. As there was negligible fatigue crack growth at the mid-thickness (and thus a non-uniform crack length) in spite of the PWHT, ~10% of the CTOD specimens did not achieve the validity requirements of BS 7448:Part 2; however, the technical invalidity of these data has been ignored, and they have been included in the transition curves ¹ . These (through-thickness) fracture toughness data reveal that the addition of 1% Co to the base-line composition improved toughness for both the 9%Cr and 10%Cr series. The larger additions of Co accompanied by the removal of Nidid not give any significant improvement in toughness.

Table 6 Summary of CTOD transition curve data

1 Hadley and Dawes ^[10] showed that no convincing evidence of an increase or decrease in measured fracture toughness of non-conforming specimens is recognised with respect to a/W ratio and crack front straightness, but consistently higher CTOD values are often associated with weld metal specimens having insufficient crack growth from the machined notch.

Discussion

The Effect of Composition on Microstructural Development, Hardness and Tensile Properties

The compositional variations studied did not significantly affect the transformed microstructure, even with the addition of ~3%Co, either as-welded or after a PWHT. In all cases the volume fraction of retained δ-ferrite was extremely low (<1%), too low for meaningful point-counting. Earlier TWI work on modified 9%Cr-1%Mo weld deposits suggested that no appreciable δ-ferrite (as recorded by point-counting) would be retained for chromium equivalent (Cr _eq ) values less than 8 ^[4] . It appears that this statement is equally valid for the current W-containing deposits, which all had Cr _eq values of <7.5.

The main microstructural difference observed was in the extent of the delineation of the grain structure, this being most evident in the 9%Cr-series deposits following PWHT. This is probably mainly attributable to greater carbideprecipitation, and perhaps greater grain boundary segregation compared to the 10%Cr-series deposits by virtue of the higher C, Mo, Nb and V contents. The grain boundary delineation, and in particular the presence of a light-etching grain boundary phase, may also be influenced by the transformation behaviour, and could be due to the retention of fine δ-ferrite colonies during the δ-ferrite to austenite transformation. ^[3] The apparent absence of pronounced delineation in the 10%Cr-series deposits may result from a lesser degree of precipitation following PWHT, and the high level of Cu, in combination with lower Mo, compared with the 9%Cr-seriesdeposits, suppressing the retention of fine δ-ferrite.

In general, the Vickers hardness data supported the microstructural observations showing only limited variation within and between the two series of deposits. For both the 9%Cr and 10%Cr series, the addition of 1%Co to the base-line deposit generally gave a hardness and strength increase after PWHT that was lower than other deposits in the series. For both deposit series, the base-line deposit exhibited the highest hardness after PWHT. This can, at least in part, be explained from the minor compositional variations, mainly the higher levels of C, Mn and W. For both series, the limited tensile data, and the inevitable scatter, do not reflect any further trends for 0.2% proof strength or tensile strength with changes in composition. As noted earlier, in all cases the values recorded exceeded the ASME Code Case requirements.

The effect of composition on toughness

The Charpy toughness measured for the root subsurface location was generally lower than that for the cap subsurface region. This is believed to arise primarily from the higher strain in the root, but may also reflect small differences in dilution and microstructural sampling. The poor lower bound toughness recorded for the base-line deposits, at least in part, reflects the higher hardness of the deposits after PWHT, to which higher levels of elements such as C, Mn, V, Ti, W and Cu probably contributed. Adding 1% Co increased toughness after PWHT for both the Charpy and CTOD data, reflecting the lower hardness of these deposits. It is not, however, clear why an increase in the Ni content up to 0.87% in the 9%Cr 1%Co W-containing deposit (W13), did not give a further improvement in toughness, as perhaps expected from earlier Charpy data. ^[4]

No toughness requirements for either the base steels or weldments are given in the ASME Code Cases. However, as outlined in Ref. ^[1] , a target of ≥40J at 20°C was set for the parent steels. As evidenced from the published literature ^[2,12] the NF616 and HCM12A parent steels exceeded this requirement. However, the weld metal toughness at 20°C was typically only ~20-30J in the present study, i.e. below the parent steel requirement. Comparison with data generated previously on weld metals for grade 91 ^[3,4,11] revealed that for the same heat treatment condition (PWHT 2hrs at 760°C) the toughness of the W-containing deposits was consistently inferior to the W-free deposits, reflected in a 20-40°C higher temperature to achieve0.1mm CTOD and 40-60°C higher temperature for 40J absorbed Charpy energy.

The effect of 1-3%Co on the Charpy toughness of weld metals from grade 91, as assessed by one of the consumable manufacturers, was presented as part of an earlier TWI study. ^[4] In contrast to the present work, the data showed a deterioration in toughness (based on temperature to achieve 40J) for the addition of 1 or 3%Co, but a minor improvement for a 2%Co addition. However, where 1%W was present inthe deposit, the addition of 1 or 2% Co improved the Charpy toughness, but both were slightly less effective than the addition of 1%Ni. This earlier study also showed that increasing Co gradually decreased proof and tensile strength, whereas there was little variation in tensile properties in the present study.

The lower bound Charpy and CTOD data sets for the present W-containing deposits have been analysed, with regression analysis carried out to allow the effect of composition on Charpy impact toughness and fracture toughness, and the interactive effect of elements to be studied. In view of the small data set, a stepwise regression analysis was carried out, using Microsoft Excel. Three equations were derived in terms of composition, one for the 40J Charpy temperature at each of the cap and root subsurface locations and one for 0.1mm CTOD temperature:

C _v cap: T _40J = - 334Mn - 119Ni - 30.1Co + 597Mo + 1705W -7929N - 1785    (1)
C _v root:T _40J = - 391Mn - 131Ni -48.8Co + 798Mo + 1659W - 8037N - 1674    (2)
CTOD: T _0.1 = - 432Si - 19.5Ni + 22.6Mo + 833Nb + 531W -1942N - 555    (3)

The equations show a reasonably good fit to the experimental data, as indicated by values of 'R ² ' close to unity; however, it should be recognised that the data sets are only small, several elements inevitably vary at the same time, and composition ranges are all small. MINITAB (release 12) statistical software was also used, in a number of ways, including forward and backward regression, to permit assessment of the existence and significance of second order element interactions, but no second order interactions were identified.

Co and Cu were each found to have only a small (beneficial) effect on toughness, and so were excluded from further analysis, in order to allow consideration of the effects of other elements. The equations indicate consistent trends for all the elements considered. Mn, Ni and Co were beneficial to Charpy toughness; however, as Mn and Co did not show strong correlations for CTOD, they were removed from the analysis, with Si being found to be beneficial. The elements Mo, Nb and W had a detrimental effect on toughness, probably through the formation of carbides during PWHT. The detrimental effect of Nb is in agreement with earlier Charpy and fracture toughness data generated in studies by TWI and others on Grade 9l weld metals ^[3,13,14] . The beneficial effect of N is contrary to earlier TWI data. ^[11]

It has been shown from this programme that adding 1%Co to the base-line compositions improves toughness. However, these weld metals are designed for elevated temperature service, and it is clearly essential that the effect of the compositional variations on creep is also evaluated. The creep rupture properties have been evaluated within the COST 522 programme.

The relatively low CTOD values obtained at near ambient temperature may initially raise concern about the defect tolerance of these deposits. However, as shown for grade 91 weld deposits, ^[15] as PWHT is mandatory and will lower residual stress levels, the calculated defect tolerance at the hydrotest temperature is expected to allow a defect size that should be readily detected with conventional inspection techniques. This would need to be verified for the W-containing deposits.

Summary and conclusions

A programme of welding, heat treatment, microstructural examination and mechanical testing has been carried out on the 9 to 11%Cr, W-containing weld deposits with controlled compositional variations, primarily in Ni, Co and Cr. The following conclusions can be drawn for the specific welding and heat treatment conditions and compositional ranges studied:

1. Highest toughness (40J Charpy temperature and 0.1mm CTOD temperature) after PWHT for 2 hours at 760°C was obtained for the addition of 1%Co to each of the base-line weld metal compositions.
2. The compositional variations studied did not significantly alter the transformed microstructure for either the as-welded or PWHT condition. All deposits had a predominantly martensitic microstructure, with, in some instances, isolated colonies of retained δ-ferrite.
3. The compositional variables studied did not significantly affect the weld metal ambient temperature tensile properties after PWHT.
4. The toughness of the W-containing weld deposits was consistently inferior to the W-free deposits studied earlier, reflected in a 20 to 40°C higher temperature to achieve 0.1mm CTOD and 40 to 60°C higher temperature for 40J absorbed Charpy energy.

Acknowledgements

This work was funded by the Industrial Members of TWI and, in part, by the UK Department of Trade & Industry 'Cleaner Coal Technology R & D Programme'. The support of numerous colleagues at TWI is gratefully acknowledged. Metrode Welding Products Limited provided the experimental electrodes, and Sumitomo Metal Industries and Nippon Steel Corporation supplied the parent steels and base-line consumables.

References

1. Metcalfe E & Bakker W T, 'EPRI Project RP1403-50 Advanced 9Cr/12Cr Materials for Thick Section Boiler Components', New Steels for Advanced Plant up to 620°C, Proc. EPRI/National Power Conference, (London), May 1995 pp1-7.
2. Naoi H, Mimua H, Ohgami M, Morimoto H, Tanaka T, and Yazaki Y, 'NF616 Pipe Production and Properties and Welding Consumable Development', New Steels for Advanced Plant up to 620°C, Proc. EPRI/National Power Conference, (London), May 1995 pp8-29.
3. Barnes A M, 'The influence of composition on microstructural development and toughness of modified 9Cr 1Mo weld metals', TWI Members' Report 509/1995, May 1995.
4. Barnes AM, 'Effect of composition and heat treatment on the microstructure and mechanical properties of modified 9Cr 1Mo weld metals', TWI Members' Report 534/1995 December 1995.
5. Patriarca P, 'US advanced materials development program for steam generators', Nuclear Tech. (1976) 28(3) 516-536.
6. Kaltenhauser R H, 'Improving the engineering properties of ferritic stainless steels', Met. Eng. Quarterly (May) '71 11(2) 41-47.
7. Morimoto H, Tanaka T, Yurioka N, Mimura H and Miyake T, 'Welding consumables and mechanical properties of weldments for 9Cr 1-8W steel for USC Boiler Piping and Tubing Applications', Proc. CSPE-JSME-ASME International Conf on Power Engineering 22-26 May 1995 Vol 2 pp 1076-1081
8. Tanaka T, Sakurai H, Morimoto H and Miyake T, 'Development of welding consumables for USC boiler tubes and pipes', Materials for Advanced Power Engineering, Part 1 Eds. Coutsouradis D. et al pp395-404.
9. Sawaragi Y, Iseda A, Ogawa K, Masuyama F and Yokoyama T, 'HCM12A pipe production, properties and welding consumable development', New steels for advanced plant up to 620°C, Proc. EPRI/National Power Conference, (London), May 1995 pp45-55.
10. Hadley I and M G Dawes, 'Fracture toughness testing of weld metal: Results of a European Round Robin', Fatigue and Fracture of Engineering Materials and Structures Vol 19 No 8 pp 963-973 1996
11. Barnes AM, 'The effect of manganese and nitrogen on microstructure and toughness of modified 9Cr1Mo weld metals', TWI Members' Report 628/1997, October 1997.
12. Sawaragi Y, Iseda A and Ogawa K, 'HCM12A Pipe Production Properties and Welding Consumable Development', New Steels for Advanced Power Plant up to 620°C, Proc. EPRI/National Power Conference, London, May 1995 pp 45-55.
13. Panton-Kent R, 'Weld metal toughness of MMA and electron beam welded modified 9Cr1Mo steel', TWI Members Report 429/1990, November 1990.
14. Dittrich S and Heuser H, 'SMAW of P91 Piping with optimised filler metals', Presented at 73 ^rd Annual AWS Convention, (Chicago), 1992.
15. Zhang Z, Farrar J C M and Barnes A M, 'Weld Metals for P91 - Tough Enough?', Welding & Repair Technology for Power Plants, Proc 4 ^th EPRI Conference, (Naples), Florida 7-9 June 2000.

Industrial Members may access further details on this topic in TWI Research Report 744/2002 Barnes A M: The effect of composition on microstructural development and toughness of weld metals for advanced high temperature 9-13%Cr steels, May 2002.