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Available online at www.rilem.net Materials a n d Structures 38 (April 2005) 353-357

Effect of normalizing heat treatment on the mechanical behaviour of low-alloy steel weld metals

V. B. Trindade l, J. C. Paygo-Filho l, A. S. Guimar~es 1 and R. P. R. Paranhos 2 (1) Programa de Engenharia Metalfirgica e de Materiais, Universidade Federal do Rio de Janeiro (COPPE/UFRJ), Rio de Janeiro (RJ), Brazil (2) Laboratdrio de Materiais Avangados, Universidade Estadual do Norte Fluminense (UENF), Campos dos Goytacazes (RJ), Brazil Received: 5 January 2004," accepted 29 September 2004

ABSTRACT In equipment manufacturing there are occasions when the base metal need to be hot or cold worked prior to welding. After welding, the components have to be submitted to a normalizing heat treatment in order to recover the original mechanical properties. In this work, low alloy steel weld metals have been studied in the as-welded condition and after normalizing heat treatment. It was observed a high decrease of the tensile properties after normalizing. The toughness increases after normalizing heat treatment, except for one weld metal where a great content of martensite-austenite-bainite constituent was formed. 1359-5997 9 2004 RILEM. All rights reserved. RI~SUMI~ Darts la fabrication d'~quipements, il y a des moments oil le mdtal non pr~cieux a besoin d~tre travailld d chaud ou glfroM avant soudage. Aprbs saudage, les composants doivent Otre soumis gl un traitement thermique de normalisation afin de rOcupdrer les propridtds m~caniques originales. Dans ce travail, des aciers faiblement allids par soudure ont dt~ dtudids pour les bruts de soudage et aprbs normalisation du traitement thermique. On a observO une forte diminution des propriOtds de tension aprbs normalisation. La rigiditO augmente aprbs normalisation du traitement thermique, sauf pour un mdtal de soudure oir est apparue une importante teneur en martens#e, austOnite et bainite.

1. I N T R O D U C T I O N The weld metal metallurgy for CMn and low alloy steels differs significantly from the base metal metallurgy in several aspects: heating and cooling rates of a weld are much more faster than submitted to a steel base metal during its manufacturing process; the microstructure of the weld metal is columnar and as melted, and not submitted to any subsequent thermo-mechanical treatment; weld metal carbon content is usually kept below 0.10%, while CMn and low alloy structural steels have 0.12%C-0.25%C; it is observed that CMn and low alloy steel weld metal microstructure is a complex mixture of two or more constituents (proeutectoide ferrite, polygonal ferrite, aligned and non-aligned side plate ferrite, ferrite-carbide aggregate and acicular ferrite) [1]. When alloying elements are added to the weld metal, upper and lower bainite, martensite and the A - M (austenite with martensite) microconstituent may be formed [2, 3]; tensile properties of the weld metal is relatively high when compared to a base metal of similar chemical composition.

1359-5997 9 2004 RILEM. All rights reserved. doi:10.1617/14177

The manufacturing of equipments by welding usually require cold conformation of steel plates and for few cases hot forging in order that plates acquire specific shapes. When working with thick plates, welding generates a high level of residual stresses, and it is usual to perform a stress relieve heat treatment after welding. This is always done at temperatures between 600~176 well below Acl, and then it does not change significantly the microstructure and mechanical properties of both base metal and weld metal. For some few cases, when the steel is submitted to a high degree of cold working or when the steel is hot worked, it is necessary a normalizing heat treatment in order to recover the original mechanical properties of the base metal. Due normalizing involves heating above Ac3 in order to promote ferrite recritalization on the base metal, this will change the original characteristics of an as welded structure. The effect of the normalizing heat treatment on the weld metal has not been yet well studied on the literature. So, the present work has as objective to evaluate changes at microstructure and mechanical properties of CMn and low alloy steel weld metals after normalizing heat treatment.

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V.B. Trindade et aL /Materials and Structures 38 (2005) 353-357

2. E X P E R I M E N T A L PROCEDURE

Table 1 - Chemical composition and carbon equivalent (CE) of the weld metals Chemical composition (wt. %) Weld metal

S Mo Cr Ti B CE C Mn Si P Weld metal joints were 0.29 obtained with submergedCMn 0.08 1.63 0.40 0.022 0.015 . . . . . . . . . arc welding, and the CMnMo 0.05 1.70 0.23 0.024 0.021 0.50 . . . . . . . 0.42 adopted welding procedure CMnMoTiB 0.08 1.58 0.31 0.021 0.016 0.48. - 0.012 0.0018 0.44 was according to AWS CMnMoCr 0.05 1.25 0.32 0.024 0.015 0.52 1.06 0.52 A.5.23-97. As consumables, CE = %C + %Si/24 + %Mn/6 + %Cu/t5 + %Ni/40 + %Cr/6 + %Mo/4 + (%Nb + %V)/5 + 10%B it was used a neutral flux and four different wires: Table 2 - Results of the quantitative metallography on weld metals CMn (1.2%Mn), CMnMo (1.7%Mn, 0.5%Mo), for the as welded condition, where AF = acicular ferrite, PF(G) = CMnMoCr (0.5%Mn, 0.5%Mo, 1.3%Cr) and proeutectoide ferrite, PF(I) -- polygonal ferrite, FS(A) = aligned side CMnMoTiB (1.2%Mn, 0.5%Mo, 0.16%Ti, plate ferrite, FC = ferrite-carbide aggregate; and weld metal ferrite 0.010%B). grain size (GS) after the normalizing heat,treatment The normalizing heat treatment was made

heating the welding joints at a rate of 200~ Weld metal from ambient temperature up to 920~ holding at this temperature for 2h. Afterward, the CMn welding joints were removed from the furnace and cooled in air. CMnMo Quantitative metallography was carried out CMnMoTiB using optical microscopy taking into account a CMnMoCr high statistic accuracy according to [4]. IIW382-71 was used to identify and classify weld metal microstructure. For the normalized condition the average ferrite grain size was measured by the intercept method. The ASTM number was calculated according to Voort equation [4]. Scanning electron microscopy (SEM) was used in order to analyse microphases in weld metals in both conditions, as-welded (AW) and after normalizing heat treatment (N). Mechanical testing (tensile and Charpy V) were performed for both as-welded and normalized condition. The Charpy V impact testing was performed at three different temperatures (-20~ 0~ and +30~ 3. R E S U L T S 3.1

As-welded

GS

Constituent content I%] AF

PF(G)

PF(I)

FS(A)

FC

25 42

32 17

14 19

27 20

2 2

20 23

8 8

76

12

5

5

2

25

7

44

9

8

36

3

23

8

~tm ASTM

AND DISCUSION microstructure

Table 1 shows the chemical composition and the carbon equivalent for the four weld metal joints. Carbon, manganese, silicon, phosphor and sulphur contents are considered to be relatively constant for the range studied. The effect of alloying additions on the weld metal microstmcture can be seen with the quantitative metallografic results shown in Table 2. The CMn weld metal showed a great content of proeutectoid boundary ferrite (32%), 14% polygonal ferrite, 27% aligned side plate ferrite and only 25% of acicular ferrite. A typical photomicrograph of these weld metal constituents is shown in Fig. 1. The addition of 0.50%Mo (CMnMo weld metal) promoted the increase in acicular ferrite to 42% and polygonal ferrite to t9%, while the others ferrite morphology types were significantly reduced (except for FC), showing the effect of Mo promoting a microstmcture refinement. The addition of Ti-B (CMnMoTiB weld metal) promoted an additional increase of acicular ferrite content to 76%, showing the strong effect of Ti and B addition

Fig. 1 - Optical micrograph of the CMn weld metal in the aswelded condition showing acicular ferrite (AF), proeutectoide ferrite (PF(G)), polygonal ferrite (PF(I)) and aligned side plate ferrite (FS(A)). on microstmcture refinement. The ability of B to promote acicular ferrite is well known [5] to be dependent of Ti presence, which combines with both C and N of the weld metal, releasing B to segregate at austenite grain boundary, and than reducing the austenite-ferrite transformation temperature. The effect of 1.06%Cr addition can be observed comparing CMnMoCr with CMnMo weld metal. Although acicular ferrite content of these weld metals are almost the same (44% and 42% respectively, see Table 2) it can be observed the effect of Cr increasing aligned side plate ferrite and reducing polygonal and proeutectoid ferrite. This is also attributed to the higher Mn content of CMnMo weld metal (Table 1), because Mn is considered to be more effective than Cr to reduce the austenite-ferrite temperature transformation.

V.B. Trindade et aL / Materials and Structures 38 (2005) 353-357

355

Fig. 3 - Optical micrograph of the CMnMo weld metal after the normalizing heat treatment showing equiaxial felTitegrains and ferrite-carbide aggregate.

Fig. 2 - Scanning electron micrograph of the weld metal CMnMoTiB in the as-welded condition showing the A-M (austenite-martensite) microconstituent. For all weld metals, the amount of ferrite-carbide aggregate remained in the range of 2%-3%. In fact, optical metallography does not have enough resolution to correctly identify microconstituents of the weld metal, and this becomes clear only with the aid of SEM analysis. SEM analysis showed that CMn weld metal has only ferritecarbide as microconstituent. For all others three weld metals the A-M microconstituent was identified, as shown in Fig. 2. A qualitative analysis showed more quantity of AM microconstituent for the CMnMoCr weld metal, followed by CMnMoTiB and CMnMo weld metal. It is known [6] the harmful effect of the A-M microconstituent on weld metal toughness.

(a)

3.2 Normalized microstructure For the weld metals studied, the original as-welded microstructures (Fig. 1) was changed to an equiaxed ferrite microstructure with ferrite-carbide aggregates. Fig. 3 shows a typical optical microstructure of the CMnMo weld metal after the normalizing heat treatment. The complete anstenitization of the weld metal imposed by the normalizing heat treatment, associated with its significantly lower thermal cycles when compared with the welding thermal cycles, generates a coarser equiaxed ferrite when compared to the as welded microstructure rich in acicular ferrite. The grain size of the normalized equiaxed ferrite is shown in Table 2. It can be observed that the grain size of equiaxed ferrite is kept relatively constant for all weld metals, with 20gm for the CMn weld metal and 231.tin 251.tm for the three low-alloyed weld metals. SEM was used to identify ferrite-carbide aggregates observed by optical microscopy after weld metal normalization. For the CMn weld metal, perlite and cementite film at the equiaxed grain boundary ferrite was obtained (Fig. 4a). For the low alloy weld metals, Fig. 4b

(b) Fig. 4 - Scanning electronmicrographof normalizedweld metals showing (a) perlite and cementitefilm at ferrite grain boundary (CMn weld metal), and (b) M-A-B (CMnMoTiBweld metal).

V.B. Trindade et al. / Materials' and Structures 38 (2005) 353-357

356

shows an aggregate consisting of three constituents: martensite, retained austenite and bainite. This was also observed by Evans [7], who called it as M-A-B constituent. 3.3 M e c h a n i c a l

3.3.1

properties

Table 3 - Mechanical properties of the weld metals in the as-welded and normalized conditions, where YS = yield strength in MPa, UTS = ultimate tensile strength in MPa, El. = elongation (%) and RA = reduction of area (%); and Vickers microhardness (HV 0.1) Mechanical properties

Vickers hardness

Table 3 shows weld metals Vickers hardness for the as welded and normalized conditions. For the as welded condition, hardness was 170 HV 0.1 for the CMn weld metal and 180 HV 0 . 1 - 198 HV 0.1 for the low alloyed weld metals. After normalizing, CMn weld metal was the only weld metal which experienced a significant drop in hardness (51 HV 0.1), while for the low alloyed weld metals hardness remained almost with the same values as the as-welded condition. This is attributed to the solid solution strengthening due to the addition of alloying elements (Mo, Cr, Ti and B) and the formation of M-A-B constituent at the low alloyed weld metals.

3.3.2 Tensile properties Table 3 shows the mechanical properties for the four weld metals in the as-welded and normalized conditions. For the as-welded condition, yield and tensile strength increase in the following order: CMn, CMnMo, CMnMoTiB and CMnMoCr. These results are consistent with the degree of alloying addition in the weld metals (which can be measured by the carbon equivalent values shown in table 1) and the associated solid solution strengthening. In addition, low alloyed weld metals have a higher content of fine acicular ferrite, which has a high dislocation density and high angle grain boundary. Elongation and reduction of area, as expected, showed opposite behaviour, i.e., have been reduced while carbon equivalent has increased. For the normalized condition, table 3 shows a remarkable drop in yield strength when compared to the aswelded condition, although the tensile strength also has decreased for all weld metals. This is attributed to the austenitization and low cooling rates characteristic of the normalizing heat treatment, producing a matrix of coarse equiaxial ferrite. It is known that the as-welded metal and acicular ferrite have a high dislocation density which combined with the small grain size of the acicular ferrite produces a considerable high yield and tensile strength. Elongation and reduction of area, as expected, showed opposite behaviour, i.e., have been increased while yield and tensile strength have reduced.

3.3.3 Charpy V toughness Fig. 5 shows Charpy V results for the weld metals in the as-welded and normalized conditions. For the as-welded condition, Charpy V energy at -20~ is higher for CMn weld metal, followed by CMnMoTiB, CMnMo and CMnMoCr. Only this last one presents results near the minimum threshold of 27 J usually required for pressure vessel manufacturing. It is known in the literature the beneficial effect of acicular ferrite on toughness [6],

Weld metal

YS

UTS

El.

RA

HV 0.1

434 500 545 565

585 605 620 685

30 25 27 24

72 66 63 58

170 194 180 198

305 244 241 248

448 472 454 510

38 36 36 33

74 70 74 61

119 186 179 205

As-welded CMn CMnMo CMnMoTiB CMnMoCr

Normalized CMn CMnMo CMnMoTiB CMnMoCr

300

(AW) . . . . --ll-- CMn 250- 0-- CMnMo --&-- CMnMoTiB 200- --V-- CMnMoCr

(N)' ' ' [] CMn O-- CMnMo --A-- CMnMoTiB / Z X --V ~ l : l

150 100-

50~ 0

V~

-i0

;

v

1'0

2'0

3'0

T e m p e r a t u r e [~

Fig. 5 - Charpy V energy of the weld metals in the as-welded (AW) and normalized (N) conditions for three different temperatures (-20~ 0~ and +30~ although in the present work CMn weld metal showed the lowest content of acicular ferrite among all weld metals studied (Table 2) and the greater values of impact toughness. The lower toughness of the low alloy weld metals is attributed to the presence of A-M microconstituent on the as-welded microstructure, as mentioned earlier, and to its harmful effect on the toughness of the weld metal. For the weld metals in the normalized condition, Fig. 5 shows that Charpy V energy at -20~ is higher for the CMn weld metal, followed by CMnMo, CMnMoTiB and CMnMoCr. The last two weld metals present results below the threshold of 27 J, limiting its use for the normalized condition. The low toughness values are attributed to the presence of M-A-B constituent observed for the three low alloyed weld metals which, similar to the A-M constituent observed in the as-welded condition, is considered to be harmful to toughness. The results obtained in the present work allow to choose welding consumables for both the as-welded and after

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V.B. Trindade et al. / Materials and Structures 38 (2005) 353-357

normalising condition. It is supposed that steels used for pressure vessel manufacturing usually have 470 MPa minimum tensile strength and impact toughness greater than 27 J at -20~ In this case, for the as-welded condition, CMn wire is considered as the better choice. For the normalized condition, CMnMo weld metal is considered to obtain the better compromise between toughness at -20~ and tensile strength. Despite it, attention should be given to the low yield strength obtained for all weld metals, because for many engineering purposes yield strength is the required property used in the design of equipments.

4. CONCLUSIONS From this work it is possible to draw the following conclusions when evaluating the effect of normalizing heat treatment on weld metal properties: 1.

2. 3.

4.

the original as-welded metal, fine-grained microstructure is changed to a coarse-equiaxed ferrite with ferrite-carbide aggregates; yield and tensile strength properties are considerable reduced; low-alloyed weld metal (CMnMo, CMnMoTiB and CMnMoCr) developed M-A-B constituent, which impaired toughness at low temperatures; CMnMo weld metal presented the best compromise between tensile strength and toughness at low temperature.

ACKNOWLEDGEMENTS FAPERJ, CAPES and CARBOOX acknowledged for financial support.

are

gratefully

REFERENCES [1] A16, R.M., Rebello, J.M.A. and Charlie, J., 'A metallographic technique for detecting martensite-austenite constituents in the weld heat-affected zone of a micro-alloyed steel', Materials Characterization 37 (1996) 89-93. [2] Matsuda, F., Fukada, Y., Okada, Shiga, H.C., Ikeuchi, K., Horii, Y., Shiwaku, T. and Suzuki, S., 'Review of mechanical and metallurgical investigations of martensite-austenite constituent in welded joints in Japan', Welding in the World 37 (1996) 134152. [3] Evans, G.M., 'Effect of manganese on the microstructure and properties of all-weld metal deposits', HW/IIS Doe. II-A-4 (1998) 32-77. [4] Voort, V. and George, F., 'Metallography: principles and practice', 1st Edn., (McGraw-Hill Book Company, USA, 1984). [5] Ortega, L.P.C., 'Effect of addtition of Ti and B on the toughness of weld metals obtained by submerged arc-welding - only available in Portuguese', (M.Sc. Thesis, Federal University of Rio de Janeiro, Brazil, 1999). [6] Ortega, L.P.C., Pay~o Filho, J.C. and Paranhos, R.P.R., 'Use of experiment planning in microstructural and toughness analysis of weld metals obtained by submerged arc-welding', Soldagem & Inspeq6o 12 (1999) 1-11 [only available in Portuguese]. [7] Evans, G. M. and Baily, N., 'Metallurgy of Basic Weld Metal', 1st Edn. (Abington Publishing, Cambridge, London, 1997).