Tmcp Steels and Their Welding 12342

8/18/2019 Tmcp Steels and Their Welding 12342

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o re welding m the Worldflx Soudagc dan, Ie Monde Vol 15 No 6 pp 17B.-1lj() 199S( J Pergamon Copyright o It IIW/IIS

rg Pnntcd m Great Bntam All nghl’ 1’(-er.edl1mpnme en Grdnde Bretagne El%c ier S, ienle Lid’ Elxwer Smemc Lld(1(141-==lihWS S9 51) .0(I(1

W43-22U(95)OOM-8

TMCP steels and their welding

B)S A

/by N Yunoka(Japan)>1. Introduction contmuous hot stnp mill was successfully employed m the UK

[2) This was the ongma) technique of on-fine accelerated coohngTMCP (Thermo-Mechamcal Control Process), whtch is a process (AcC) immediately followmg hot rolhngof controlled hot rolling followed by on-hne accelerated cooltng,was first employed to produce steel plates m 1980 Smce then. TMCP (the rmo- mechanical control process) is a combined tech-many types of steels, mcludmg a sheet steel, a shape steel and a nology of CR and AcC This technology was first mtroduced onstainless steel, have been manufactured by TMCP The accelera- an mdustna) scale to manufacture steel plates m 1980 m Japanted

cooling strengthenssteels and can reduce the use of harden-

TMCP makes it possible to concurrently control not only the

able alloying elements. resultmg in hardenability reduction and coohng rate but also the start and halt temperatures of water cool- thereby the improvement of weldablhty However. TMCP steels 109 It thus can control the compostnonal ratto of the transfonna-

somewhat differ from conventional steels 10 chemical compost- tion products such as femte, pearhte, baintte and martensite at auon and they are sensitive to some types of heat treatments desired level and produce steel plates with the preferable balance

of strength and toughness Because of its capabihty of producmgVanous new features have ansen in the weldmg of TMCP steels steels with a flexible selecuon of steel mechamcal properties,They include HAZ hardness. HAZ softemng, cold crackmg. HAZ TMCP has been able to manufacture steel sheets, shape steels andtoughness, post weld heat treatment, weld metal toughness, and statnless steels other than the platessolidification cracktng All of these items are more or less relatedto HAZ hardenabtltty Therefore, this review discusses the items 2 2 Platefocussmg on HAZ hardenability The objective of this review is toa


2/16

,

376 TMCP STEELS ANDTHEIR WELDING

,N RO

Oi

,

-m [A AC3 AC3:; -.- -Ac3 --- -- AC3

-

Ar3 Ar r 3 Ar3 T

-

lE § Arl Arl

_ - Ac, Arl

------- Ac,

¡ j) A

A&dquo;B&dquo; A&dquo;B’3(’

250 -formed by DQ from the femte-and-austenite dual phase zone250 -

1I

1I I

II 1

I.I, (between Ar, and Ar, transformation temperature) This type of

.32 36 40 44DQ is called DLQ In DLQ steels, soft femte phases first yield in

CE=C+Mn/6+(Cr+Mo+V)/5+(NI +Cu)/15 tensile testing while lowenng the yeld strength, and then hard

Fig 2 Relation between carbon equivalent and steel strength of TMCPbainite phases resist ductile fracture, elevating the tensile

and conventional steels [81

c n equ


3/16


4/16

378 TMCP STEELS AND THEIR WELDING t

2 l Sramless steel

Mn Cr+Mo+V Ni+CuCE =C++---- AUlemtlc stainless steel,, are subjected to the carbides solid olu- 500 -CE =C+ 6+ 5 + 15

tion heat treatment for improvement of their corroson resistance_

However, soltd soluuon treatment reduces steel strength and-

austenitic stainless steels are, thus, not appropnate for use as-

structural members TMCP techniques have been employed to-

o ;

strengthenstainless steels

without reducing thecorrosion tests-

’S&horbar;o**’’- _ CE=0 43--e‘_:

tance propentes Since austenitic stainless steels do not transform o 400 -B 8 - - - - - - - -dunng hot rolhng, strengthemng by low temperature transforma- ; f¡ iuon products is not expected However, controlled rolhng at the. CE = 0 38 0non-recrystalhzlOg temperatures facilitates grain refinement caus- 0 &dquo; ting steel strengthening Moreover, accelerated coohng retards

-

.*&horbar;&horbar;&horbar;&horbar;-** ’&horbar;&horbar;carbide precipitation so that a soltd soluuon treatment canbe

CI- B *8

CE=0.37 64,

omitted for TMCP au stem tic stainless steels 131 E 300-B o CE = 0 33 0’C)

300 - sbCE= 13’

25 Shape steel 0o*

CE=036cW

Shape steels including wide flange beams and ruls are formed 10 - w CE = 030not simple section shapes, and thus a uniform thermal control m - , _ __’***%*hot rolltng and accelerated coohng is difficult However. TMCP 200 - CE = 0.28

asbeen introduced to manufacture

shapesteels [ l4] For

_

,

mstance, the employment of TMCP is desired to improve the-

, ,i

toughness of a wide flange beam core where a flange and a web 10I

I 30I

I 50 I II I

100I

130I

II

intersect In convenaona( hot rolhng, crystalhne grains coarsenBead Length (mm)

!because of the high hot rolhng fimshmg temperature due to the

Bead Length (mm):

heavy secuon at the core. resulung in significant deterioration ofFig6 Relation between bead length and EtAZ maximum hardness 171 :

1

toughness at the core portton ,i

Rails, 10 general, consist of pearhte microstructures with a eutec- building However, those shorter than 50 mm but longer than 10.

toid carbon content of 0 8% Pearlite is charactenzed by the htgh mm became perrrussible provided fabncators use TMCP steels of ,abrasion resistance needed on the heads of rails Controlled cool- TS490 MPa grades wtth CE&dquo;w, (IIW carbon equivalent) not higher

’

tng is required after hot rolhng 10 order to obtain the full pearhttc than 0 36% The hardness was still hmned to be not more thanstructures with fine lamellar spaces In fact. rapid coohng results Hv400 (Vickers hardness) and arc stnkes were not allowed evenin martensite formation and. conversety. slow coohng forms when ustng TMCP steelscoarsened pearhncstructures On-hne controlled

coohngwhich

utlhzes the latent heat of hot rolling has been developed for rail For the avoidance of cold cracking, the HAZ hardness is oftenproduction [15] High temperatures from the latent heat can be limited to Hv300 or Hv350 in the welding fabncanon of offshoreobtained before coohng unltke reheat-and-coohng m convention- structures and ice breakers The maximum permissible HAZal hot rolltng The employment of on-lme cooling results 10 rail hardness is 22 In the Rockwell C scale (Rc22), which is equtva-heads with deeply hardened portions So far, the improvement of lent to Hv248. for steel Plpehne weld HAZs to be subjected to

I

the resistance agamst abrasion and fatigue has been a pnmary moist H,S envtronments Nevertheless. HAZ is Itkely to harden

8 concern for rail manufacturers But toughness improvement has when weldtng offshore structures and pipelmes, because lowbeen attempted, especially for rails for cold countnes. through weld heat inputs have sometimes to be inevitably employed ,controlled rolltng in the lower limit temperatures of an austenne Normahzed steels have difficulty sausfymg the HAZ hardness

’

region [16] limttatton, and consequently TMCP steels are exclusively used , Ifor offshore structures and une-pipes j ’

3. HAZ hardenability and hydrogen cracking ofTMCP steels

for offshore structures and line-pipesI,

3 1. HAZ

hardenabiliv

3I2 HAZ hardenabtliiy and hardness estimation I3 ). tZhardenublrr_v A HAZ hardens most at a region close to the fusion hne where .

3.1 1Limitation of HAZ hardness the coarsening of austemte grams occurs and hardenability resul-tantly elevates Figure 7 shows the dependence of HAZ maxi-

For two years after 1983, the Japan Shipbuilding Research mum hardness on the weldmg cooling rate or the weldmg coohngCommittee (SR193) investigated the performance ofTMCP steel time between 800 and 500*C, r,s HAZ hardnesses decrease fairlywelds, focusing on whether or not TMCP steel plates of TS490 smoothly with increasing t, in all the femtic steels includingMPa grades could be used for shipbuilding [17] As to HAZ hard- TMCP steels as shown m Fig 7 A HAZ becomes full martenstncness, the SR193 committee examined the relationship between and its hardness levels off at the highest value in the coolingthe maximum HAZ hardnesses and the bead length. Figure 6 times of tL,5 shorter than the tm (point M) in Fig 7shows the expenmental results indicating that the cooling rateincreases and thereby HAZ hardnesses increase with a decrease The hardenability does not necessanly descnbe the absolute levelin the weld bead length The HAZ hardness tends to rise more of hardness of quenched steels but it represents their Lkelthood torapidly m the lower carbon equivalent steels but their hardnesses be martensittc or the facility of martensite acquisition. Therefore, -remain at a lower level than those of the

highercarbon

equivalentthe HAZ

hardenabthty maybe denoted

by tM,the

longest coohngsteels. time by which full martensite is acquired In other words, thelonger tM is, the tugher HAZ hardenabtlity results tit! is given for

Weld beads not longer than 50 mm were not permitted in ship- hypoeutectoid steels (18]


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TMCP STEELS AND THEIR WELDING 379

&dquo;

.no014C-045m150Mn 1 2-600 -

0 -Nb Vf ree

i - 1 S0 Mn

-1 1 §

- M( tM HM)HM) Hv=HB- 2 HM 2 20 HBerctanxl B9Lab -melt Steel

10::II&horbar;&horbar;&horbar;-. W.=--.... -02-{B -

7500 - 0 HV=---8rctBnIX) L. B -10 0oo ii 500 - 0

I =Q4, c-

2

t. , - log 2tM - - - - i - 04 - - 9 0*

x> _ , X-4 oo tB - oe tMtAA2

--06- !!’

B -6-

A

-

T

-O6-°

400 - .=,.. 06 -

77 £-

4UU - D - -09-

7

--

8 i 10 - B. 0 -6300 -

° Measure !B m -t0- -6300 0 Measuredte 8(tB HB) 12 - 0 8_

m A516 Gr 70 olate B(ts We) - 12 -

0

. 1,

1

I

2

I I Uu- ’_ 0 Ot1 022 033 oa4 05

Area Occuped by Inclusiors (%)100 · &dquo;,

1

i ’

5’ 10i

o

, ’

50 too

,

50o Fig 8 Effect of steel cleanliness on HAZ hardenability [24]Coolnp Time Between 800 and 500t to(s)

Fig 7 HAZ hardness change with respect to weldmg coohng ume [) 8)1 depleuon occurs around the inclusion, resulung in the enhancementof carbon diffusion and femte transformauon around the inclusion

log t, =4 60 CEI - 2 08 ( 1 ) This is a presumed reason why steels with more sulphur hardenless in their HAZ Whereas, niobium tends to co-segregate with

Si Mn Cu Ni Cr(1 -0 16·ICr) manganese and retains m a manganese depleted zone, cancelhng CEI = CP +- + - + - + -+

+

+ the effect of manganese depletton It follows that femte nucleauon24 6 155 122 8 is not facihtated by inclusions In niobium beanng steels [25] The

effect of tnclustons of HAZ hardenabthty is complexMo- +AH (2) A prease formula to estimate HAZ maximum hardnesses is need-4 ed when determining the weldmg conditions to sausfy hardness

hrmtauon requirements or when designing the cherrncal compost-cp= C (C 0 3), Cp = C 6 + 0 25 (C > 0 3%) nons of a steel to be welded under the hardness hmitation. Many

formulas have been proposed [26-31 ] The author considers that

Equation (2) is the carbon equivalent descnbmg the HAZ harden- a formula [ 18] denved under the detailed investigation of HAZabihty. where 0I! is the hardenablhty increment due to boron and hardenability is the most reltable for a wide vanety of femucsteel cleanliness Boron significantly influences AH at a very low steels Furthermore, this formula can be extended to estimatecontent It is reported that the boron effect is more significant m HAZ hardness after PWHT [32, 33] and weld metal HAZ hard-lower nitrogen steels [ 19] TMCP steels include, in general, ness [34]reduced nitrogen for their HAZ toughness improvements Thereare some high strength steels which effectively uttlize boron to 3 13 Hardness of resistance spot rseldsincrease their strength together with the HAZ toughness improve-ment Hardenable elements are reduced in these steels, and there- Resistance spot weldmg is usually employed m automobile sheetfore the HAZ hardenability mcrement by boron is designed to be welding As a quality assurance test for spot welds. a tensile shearcancelled test and/or a cruciform tensile test are performed TSS (tensile

i shear strength) and CTS (cross tension strength) are given as a TMCP steels are improved in their base metal properties as well function of i (sheet thickness). d (nugget size) and a (base metal

as in their weldability This improvement is attnbuted to the tensile strength) as followsdevelopment of not only the process technology to control hotrolling and cooltng but also the steel making technology control- TSS = A d&dquo; t 6 [35] (3)hng micro-alloy elements precisely Therefore, TMCP steels are

generally very pureand clean It was

reportedthat clean steels CTS = B

t d. [36] (4)and especially low sulphur steels harden in their HAZs more thanconventional steels [20-23] This is based not on expenments but where A and B are constantson expenence Recently, the effect of steel cleanlmess on HAZhardnesses was tnvesttgated using laboratory-melt steels with When a nugget of the appropnate size with respect to sheet thick-varying sulphur and oxygen contents [24]. Figure 8 shows the ness is formed, rupture m tensile shear tests always occurs out-

experimental results indicating that the effect of steel cleanlmess side the nugget and TSS predicted from Eq (3) is constantlyis obviously recogmzed in steels without niobium and vanadium achieved. This rupture mode is called &dquo;plug failure&dquo; In the cruo-but not in niobium beanng steels at a steel cleanlmess of 0 lo. It fonn tensile testing. CTS predicted from Eq. (4) is also obtainedshould be noted that 0.19’o steel cleanhness means intolerably when plug failure takes place However, rupture often occursduty for structural use The effect of steel cleanliness on the HAZ inside a nugget. CTS is, m this case, much lower than that pre-hardenabihty (Afr7 is considered neglIgIbly small for the normal dicted from Eq (4) and concurrently, considerably scatters in itsstructural steels, as long as oxygen is less than 50 ppm, and sul- value It ts known that many factors including the welding condi-phur is less than 0 02% lion and the steel propentes (strength and compostuon) affect the

scattenng ofCTS values [35, 36, 37]-

Some inclusions act as femte nucleation sites dunng coohng afterwelding and reduce HAZ hardenability. Specifically, a MnS The following empmcat equation of a carbon equivalent typeinclusion plays an important role in the femte nucleatton, MnS gives the transition from the plug rupture to the in-nugget rupturetends to precipitate on an existing inclusion and manganese [37-40].


6/16

.,

380 TNCP STEELS AD THEIR %%’ELDING

Si Mn

C+ - + -+?P+.lS?0_’r (5)130 20 500 -

13,

A copper electrode function% as a heat sink m resistance spot 0 /weldmg Thus. spot welds cool rapidly, r&dquo; is reponed as short as 3& B_ r < Medium C-Mn01 10 the weldmg ofI mm thick steel sheets [41] Thts rapid &dquo;* 1 _Medmm C-Mncoohng makes a nugget and HAZ around the nugget fully marten- 400 - rstuc. whose hardness is determmed by the carbon content alone

BHowever. Eq (5) includes elements other than carbon This 1iimplies that the transition of nugget rupture mode is mfluenced m Mold steel! Ia complex manner not only by the weld hardness but also by the T

1

)HAZ width (determined by HAZ hardenability) and hardness dis- i 300 - c-rQ (tnbution [42, 43] Figure 9 shows the hardness distnbution of : / B I Low C-Cuspot welds of vanous types of steels [42. 43]

:!!I I

1(873Kxx 60s)

An austenrte highly retaining steel is produced by TMCP ThisIII

A-.-A- *1’ -&horbar;r-i-),

XLsteel has been developed for the high strength members of auto- 200 - ) N*D’ < <

:I:

The high heat mput encountered m submerged arc welding. elec- , P otro-slag weldmg and flush-butt weldmg may soften HAZs of

t6o -

°

140 -

.


7/16

·. TMCP STEELS AND THEIR WELDING 381J

mg disptjcement) at the surface notches on T-joints and buttjoints subjected to high strain of 0 40rlr .i8 CTOD In this analy-

500 - C -- Testto:>sis is not a crmcj! CTOD value representing the metallurgical 500 - H I a 17kJ/mm25 - 38mm thl&dquo;&dquo;

(0’ ’ * H)I i)7kJ/’m M-3Smmth!et

toughness but a mechdnlcal value such as elongation T5 !IOOMPo Gr-’

- 20°

50. TMCP 5_10 f-- Norm ...-

In the case of butt joints CTOD was 0 3 mm under the under- 450 - 0 TMCP Strb Norm* - 150match joints with a degree of overmatching (yeld strength of ... i 400 - r*’ cR * 0weld metal/that of base metal ) of 0 90, white CTOD was 015 j

TMep(cx-M:)- cooO

- 100

mm under the overmatchjoints

with a

degreeof

overmatchingof

a 350 -°

I 2 This means that overmatching is preferable However. the g- 300- ozo c 50analysis of T-omts with higher inherent restraint than butt Jomts = /’

revealed that CTOD is 0 3 mm irrespective of the degree of over-


8/16

’

182

125 I

TMCP STEELS AND THEIR WELDING

, i125 , , . , ,3 3 ’_ CoIJ c rucl.rng vusc eptibilir% rnder HI =17kJ/’ H&dquo;w = 5ml/100gDM IH ) -) 7kJ/mm

ys(WM)=400MPa

Smce Dearden and O’Neil 1521 published the concept of carbon 100- -

equmalent in 1940, many mdices evaluating the cold cracking E B B B B Bsusceptibility ot steels have been reported av shown m Table 1 1§ B B B B B BThey are roughly divided into three groups, the first is of a CEI, m yg-B B B B B 150 ’type which ongmated from Deardens carbon equivalent the sec- vi 125C

ot 0

ond is of a Pcm type which regards carbon as more important = B B B IOO,G B B. ,

than the first group and the third is CEN carbon eqmvalent in f- 50 _ B B

751oo*c

B B - iwhich the significance of alloy elements vanes depending on a 0 50D

75C< B B.

carbon content CEnw has been long used m steel specifications as a1B1

5’C 25a weldability yardstick CE&dquo;w well assesses cold cracking suscep- 25 -

t-

tibility of carbon steels and carbon-manganese steels, while Pcm - **-***better evaluates that of low-carbon, low-alloy steels CEN has &horbar;**- ,been proposed to assess cold cracking susceptlblhty of both car- 0 1 1 ! i 1bon steels and low-carbon, low-alloy steels It should be noted 033 04 05 066 ¡that a CE,tw type of carbon equivalent also descnbes HAZ hard- Carbon Equivalent. (CEN)enabtltty because it is similar to Eq (2) - Frg 14 An example of predicted necessary preheat temperature [661

Many methods have been proposed to determine the necessarypreheat temperature in steel weldmg [58-63] However, many hydrogen diffusion from a femte weld metal with lower hydrogen ,

a problems remain 10 their appltcanon to actual welding practice solubtltty to an austenite HAZ with higher solubtlrty On the otherFor tnstance, the same preheat for TS490 MPa steels and TS785 hand, TMCP steels are produced generally with low HAZ harden-MPa steels is given in the methods based on the Pw-t, [58], Ct- abtltty, i e , their HAZ may transform from austenite to femtet,m [61], and AWS D1-90 Appendix IX [64] cntena if their rel- earlter than weld metal This situation is opposite to the case ;evant carbon equivalents are the same The preheat given from shown m Fig 1 S, and diffusion from a weld metal to HAZ thusthe above cntena is too conservative for TS490 MPa grades and dtrrumshes [68] Therefore, weld metal cracking night be moremay be, contranly, insufficient or nsky for TS785 MPa grade ltkely m TMCP steel weldmg than expectedsteels BSS13S [65] is very convenient because necessary preheattemperatures are given in figures and tables However, this There have been some methods reported to detennme the neces-method is based on CEllw, and thus it is considered inappLcable sary preheat temperature for avoiding multi-pass weld metalfor low-carbon. low-alloy steels As a matter of fact. BSS 13S is cracks [69-71] The following is one of these [71]based on the premise of carbon steels and carbon-manganesesteels but not low-alloy steels Recently. a chart method [66] has Tp (°C) = 0 524 oB + 277 log Hoc - 482 (6) ,been proposed which is based on the CEN carbon equivalentThis method considers the effects of steel composition, weld where 6B is the weld metal tensile strength (MPa) and Hoc is weldmetal hydrogen, welding heat input plate thickness, weld metal metal diffusible hydrogen by gas chromatography (ml/100 g)>strength and joint restramts Figure 14 is one example of the nec- This procedure does not consider the effect of weld metal heightessary preheat predicted by this method (the total weld metal thickness) and this effect should be taken

into account as mentioned in the report [69]]3 33 Mull/-pass weld metal cracking

TMCP greatly improves steel weldability, while wetdabihty of a

8 Figure 15 demonstrates schemallcally how hydrogen diffuses weld metal remains unimproved because its strength and tough--from a weld metal to the HAZ dunng welding [67] This is the ness have to be provided not through a thermo-mechamcal con-case for conventtonal steels because carbon content is generally trol but under an as-welded (as-solidtfied) condition The neces-lower in weld metal than m steel and weld metal transforms mto sary preheat, therefore, should be determined based on the carbonfemte before the steel HAZ does, resulung in the enhancement of equivalent of a weld metal rather than that of a steel, especially

for high strength steel weldmg As estimated from Eq (6), weld

11 bl I &dquo;0

f .. , , , ,.....metal

crackjng hardlyoccurs in

we)d)ng ofTS490

MPasteets

¡Table t. Various types of carbon equivalent of steet weidabilityitymetal

cracking hardlyoccurs in

weldmg ofTS490

MPasteels

I

Group Formula Ref I

CE


9/16

,,


When welding TMCP steel,, with low hydrogen welding w.-,

UB _, -,Ok 1

matrnal, preheat is thu,, unnerese.trvw in TS.190 MPa Lride weld- -. 1 46 ’, ,’

; ,109 eXlept in the case of uhra-heavy thick plate welding 1:; v’

..’’ · y,

4. HAZ toughness of TNICP steels’ . - B..GBF -Oar

4 lOne-pans eld HAZ 4

The microstructure of a HAZ vane-, depending on the steel chem-ical composition and the weld heat input and HAZ fracture

toughness chinges corresponding to the microstructural change y ,Figure 16 shows the dependence of Charpy fracture appearance SP

transition temperature (I T,,) on the weld cooling time for the V.coarsened gram HAZs of one-pass welds in HAZ thermal simula- -

tion tests [72) In the small heat input welding and thereby the :short cooling time weldmg, the microstructures are of lowerbainitic (BL) and its toughness is sausfactonly high. Ie, I’T&dquo; Is :. ° ! GNvery low HAZ toughness most degrades in the weld coolingtimes (r,) between 10 and 30 s in the case of TS490 MPa grade

,

steels The microstructures consist of upper-bainite (B.) and ! .femte side plates (FSP) in these weldmg cooling times r -! .

Figure17 shows

typical examplesof microstructures at coarse

8 gram HAZs in high heat input weldmg [73] Gram boundary fer-rite (GBF) precipitates mainly along the pnor-austemte grain Iboundanes and FSP and BL grow towards inner grams fromGBF Sometimes. femte is nucleated inside grams and it is called

intragranular femte There are two types of intragranular fernte,one is polygonal ferrite (IPF) and the other is acicular fernte(IAF) When welding of high strength steel of a TS785 MPa =,grade, BL is nucleated because of their high hardenabilityHowever, when the welding heat input employed exceeds a cer-taIn level. then HAZ microstructures become upper-balmuc (BL,)

a

. ‘·,11and severe degradauon of toughness results The limitation of a F .. eweld heat input (4 IJ/mm or thereabouts) must be stnctly main-

&dquo;

tained in welding of TS785 MPa steels_

IPf ’In one-pasn welding, i T&dquo; of a HAZ increases t e HAZ toughness I-

decrease,, as weldmg heat input increases as shown in Fig 16IPF

Howeker the absolute leBel of toughness is always higher in the’

lower-carbon lower-carbon equivalent steels Figure 18 showsthe relauowhtp between the HAZ toughness In terms of the cnti-cal CTOD and CE,,, for TS490 MPa steels This relation also

implies that low carbon steels provide higher HAZ toughness T

8 although the excessive reduction of carbon results in the severe’degradauon of HAZ toughness [74] Figures 17 and 18 show that

the reduction of carbon and/or carbon equivalent is beneficial in FIg 17 Designation of HAZ microstructures [73]the improvement of HAZ toughness and it is, thus, essential to

employ TMCP which can reduce carbon equivalent or harden-

abilitywithout the reduction of the steel strength

Figure 19shows

a dependence ofthe

HAZ toughness (VT&dquo;)on

theeffective grain diameter of a HAZ mIcrostructure 10 HAZ thermal

12C- 3OS,-I.38Mn- ozrro- 36CEiiw history simulation tests of TS490 MPa steels [75] The effective350 - 0 08C- 25s,-I.39Mn- 32CEllw

&dquo;

(t:>gram sme, which is shown as d in Fig 19, is called the fracture

> A 06C- 28S,-125Mn- 15Cu- 2ON,- 29CEIIWso

facet unit, Ie, a unit step of brittle fracture propagation The

o s° toughness degradation caused by the generation of Bu and FSPE - microstructures is partly due to the coarsened size of the fracture300- -_.--o ; facet unit Since the carbon solubility is lower in femte than inI-’

-

-o austenite, carbon is expelled from transformed fernte dunng co v-

r - ----ð . transformation The thus expelled carbon tends to segregate at the

2M - I smwa.a HAZ : boundanes between laths of Bu or FSP, and the austemte-to-fer-co

- Temp 1623K(I350t:>- -60

nte transfotirtatton retards at the boundanes This results in the

Z Â--iI -50 formation of the nuxed microstructure composed of non-trans-! 200 -

, . 1.. 1.1 , ..I. IIII . I , I

I’&dquo;.

formed austemte and transformed martensitc This nucrostructure’

5 10

’ ’

so 100

’ ’

soo is called MA (martens ite-austenite constituent) MA is much_Weld Cooling Time bet- 800 .e soo>r te-5 (4) harder than the surrounding matnx and, therefore, it facilitates the

Fig 16 Dependence of Charpy ftacture appearance transition temperature initiation of bnttle fracture and lowers HAZ toughnesson welding cooling time (74) Reduction In carbon is the most effective way to dinunish MA In


10/16

, 3h4 TMCP STEELS AND THEIR WELDING I J,

350

( C )·.. v

i

~

’

I

325 - Q - 50 ‘:

300 B, 2525300 - - 25U

EE B0EE Q

0 275 Q 0 0 -0o ’0-

2750

’B.../j>0

00

°Q 0

aE

250 - - - 25---

8_ Q Q Q

Fig 20 Intragranular femte plate nucleated at Ti-oxide [75]

VI

0225 - 0 - -50 growmg IAF nucleated at Ti-oxides [81] Oxides among the IAF’

nucleating inclusions are thermally stable and maintain theircapability of fernte nucleation at HAZ close to the fusion hne

’

200 - I I I I -75over 1450*C dunng weldmg On the other hand, fernte nucleation I200 -

25 30 35 40

-75sues other than oxides melt near the fusion line Therefore, a .

- newly developed Ti-oxide dispersed steel is appropnate for high.. CE = C + M n/6 + ( Cu + N ) / 15 +( Cr + Mo + /5 heat input weld 109 as shown In the expenmental results of Fig 211 :Fig 18 Relation between critical CTOD transition temperature of HAZ (75)]and welding cooling time [74JJ

It is reponed that inclusions facilitating femte nucleation alsoeffectively prevent austerute grams at the HAZ from coarsening

Effective Gram Size. d < [80. 82] This is the so-called pinning effect The refinement of i

,&dquo;’ &dquo; ’ ’austenite grains leads to the refinement of FSP and B, growing ;

500 400 300 200 150 100 from austenite grain boundanes and thereby to the improvement’

Y 320 -I I

Q TI-O tI ,

Ct)of HAZ toughness

- Ti -B 40 4 2 Muln-pass weld HAZ.= >-

-B!:::. . T T I I - - N 8 ( 10 ppmS )

- 40 4 2 MlI.ltl-pass weld HAl

300 - 0 Ti -N( 40 ppm S )-

The microstructures of a coarsened HAZ of a multi-pass weld

E CL -Q

IAF - 201350 1400 14 50(*C) ’

9

d

I I I

2 r- 280 -d

_

2 180 -

I I I

c28° -

d

.

+ - 0180 - ,,;>=--

-

-0 I 160 --

-/’.I’ ....,-- -

_

-° - ’ 160 - ...&dquo;W 260 -

GBF 140 -to zso - _ GBF FSP - ‘20 =

140 -

I t I()(D m - FSP - -20

I I I I(*C)

- 40<

-

,... 0 Ti-O , 240 - 0 300 - !:::. TI-8 ...240 - 0

CN’

0-40

300 - A Ti-B jib

20

-

tg - 130

(;8B’- - 40

. > 0 Ti -N ,&dquo;&dquo;- 20

LL

220 - 1 te5 130S

1 1 4 1 -60 1! E 280 - ,,’,

00

00411141-60010 > 0121-2 60 0260 -- 0

o 04 0 06 0 08 010 012 260 - .’Fig 19 Relation between Charpy transition temperature and fracture I- ,,’

--20,

facet unit at fracture initiation site (75) m240 - .’ ’facet umt al fracture Imllallon site [75] 240 -&dquo;

/.

other words, it is also essential to employ TMCP from the vlew-’ - -40

point ofHAZ toughness improvement. m g 220 - A- -_ ---60Intragranular acicular fernte (IAF) is featured by its refined 0

200 I I Igrams and its fracture facet unit is always small Therefore, IAF ti 200

1623 1673 1723retains improved fracture toughness Some types of inclusions are

LL 1623 1673 1723-

considered to act as IAF nucleation sites, they may be REM oxy- Peak Temperature (K)

sulphide + BN [76], MnS + TIN [77-79], calcium oxysulphide g 21. Effect of peak temperature at HAZ on Charpy transition tempera-[80J. and Ti-oxide [75, 81Figure 20 is a microphotograph of ture and hardness [75] ,


11/16

, TMCP STEELS AND THEIR WELDING 385

change from location to location depending on the extent of the (*Cat dual phase reheated zone = C + 12 [ MA (%)j -144

mu)t)-therma!htston. Figure 22 schemaocatty shows the change id (mm)1-&dquo;-’ (7)in HAZ microstruLtures in a muttt-pass weld of a TS490 MPasteel I 1831 The HAZ whose microstructure I, coarsened by the The allo,. elements such as carbon boron and molybdenum raisefirst thermal history (the first weld) change, into different struc- HAZ hardenabt!ny and facilitate MA formation Therefore. it istures corresponding to the thermal histories of the subsequent desired to reduce these elements for HAZ toughness 1871 MAweld passes When the peak temperature of the second pass is decomposes to some extent dunng reheating under Ac, by subse-immediately above the Ac, transformation temperature. the quent weld passes because of their tempering effect Siliconmicrostructure is refined because of the so-called

normalizingeffect (Fig 22B) However when the second heat peak tempera- 50600 800 1000 1200 1400 (*C)

ture is less than Ac,. the coarsened HAZ is only tempered In the _*&dquo; ’’!’’’!’!Icase of a reheating peak temperature between Ac, and Ac, &dquo;Ic & m(austenite-femte dual phase region). the diffusible carbon gener-

-

HT490 0ated by dissolved carbides concentrates in the austemte at around - HT785 A Athe peak temperatures and hardenability nses in the austeniteSome high carbon austenite is transformed into martensite dunng Peak Temp at 1 st Cycle 1673K

cooling and MA is formed dispersively or in an island-like man- 10 - (1400t:)ner along the pnor austemte gram boundanes (Fig 22C)> r:1 8

Figure 23 shows the relation of cnnca) CTODs to the reheating g 0 5 - 8*T A Btemperature in thermal history simulation tests [841 As descnbed

-

Q B fB Babove, the cntical CTOD is the worst due to the generation of SJ

-

t B A AMA (island-like martensite) when reheated to the dual phase C

_

n B 9&dquo;’B region around 800°Cm the case of TS490 MPa steels The region * Breheated to the dual phase also exists approximately 3 mm apart re t f A Bfrom the first pass fusion hne (Fig 22E) This region is another 7ï; 01 - t fA B&horbar;locally embnttled zone due to MA other than the embnttled zones 0 -along a coarsened HAZ The HAZ microstnictures of a TS785 0: 8MPa steel with high hardenability are generally of lower bainite o0 05 -

(BL) at rather low heat input welding However, once HAZ is’

U

reheated immediately above Ac,, the microstructure changes from’

0B, to B, because of low

hardenability Dresulting from fine gram - 0

BAsize of the onginal BL structure [85] 0

The HAZ embnttlement is governed by not only the effective 0 011f I I I I

gram diameter (d) but also the total volume of island martensite 1 st Cycle1000 1200 1400 1600

(MA) The following relation represents the HAZ toughness (%,T,) Peak Temp at 2nd Cycle (K)as a function of the MA volume and the fracture facet unit, Fig 23 Relation between peak temperature of second weld pass on cnti-Imply 109 that the effect ofMA is more dominant [86] cal CTOD of HAZ [841

coarse-grain HAZ I

A I (CGHAZ)

A

,’J g ytJ j(CGHAZ)

i’ ,’,.l’I

bead3

B Fir* -grain HAZ,,’, B (FGHAZ)

bead2

BBBy c intercritically reheat-BB iS? ed coarse grain zoneB IRCG)

bead

1 ’&dquo;’-&dquo;’ ’&dquo;D

I subcritically reheated

BBBBBMS< ’’t


12/16

·. 396 TMCP STEELS ANL) I HtIR H TLDIN(,

, :retards MA decomposition because it stabihzes MA [87 981 -120 -100-80-60 -40 -20 0 20 (*C)

.

Therefore the reduction ot ,ilicon result- m the improvement of ,., I I I I I I II

toughness of mulll’pJ&dquo; H .1&dquo;Z, 1801-

;If)

- · CR-AcCDue to their fine size acicular femte (AF) can disperse carbon > A CRconcentrated zones dunng reheating to the dual phase region and _

CR (t)

then the MA formation is diminished 187) The steels effectively.340 - 0 AR // - 60

*utilizing mclusions such as Ti-oxides provide satisfactorily high 320 - // Itoughness

with their

mulu-passwelds because their HAZs

mostly g ’ -40

consist of fine IAF œ300 - +40KK /O /

E a (40*C 0 0 - 20MA forms locally embnttled zones and they are the main cause {!!. 280 - (40C) , /’1 0/

- 20

of the occasional occurrence of very low values dunng a number III / O-0

of CTOD tests It was once indicated that the TMCP steels may 260 - · O?be susceptible to local embnttlement an their HAZs This contro- .II’

--20

versy emerged presumably because TMCP was developed just 240 - i -1when CTOD testing, whtch facilitates the detection of local / · t

1 : 1 - -40

embnttlement, was prevailing and CTOD tests were thus per- J 220 - 0/ · A ./ - -60formed predommantly on TMCP steels In fact, TMCP can pre- I / ./’ - -60ctsely control the hardenability enhancing elements and theMA200 - / 523K x 3600s - -80decomposmon retardtng elements at a desired level, concurrently A (250’C x 1hr)considering the steel strength and toughness It is certam that the ti 180 - - -100

employment of TMCP manufactures steels whose HAZ is 1B ))))))I I- -100

markedly improved with respect to not only Charpy charactens- A t t t t 260 280 300

Itics but also cnuca) CTODstics but also critical CTODs

Fracture Apperance Trans Temp . vTrs (K)

5. Reheating characteristics ofTMCP steels Ftg 24 Change m base metal toughness under stram agmg (3]1

5.1Stram agingg5 3 PWHT characteristics

Steels are often subjected to reheattng mcludtng flame heattng iafter bemg cold formed to several per cent m fabncanon At this Post weld heat treatment (PWHT) including normaltztng (N),ttme, base steels degrade m their toughness due to strain agmg normalizing and tempenng (NT) and quenchmg-and-tempenng

Figure 24 shows changes in Charpy fracture appearance transition tQT) are not permitted for TMCP steels. which are strengthenedtemperatures when subjected to 250°C x 3600 s agmg treatments by accelerated cooling Hot formtng is also not allowed but warm

after 5% cold formmg [3] The degree of degradation due to forming may be performed with the necessary cauuons being

stram agmg is 20*C or thereabouts and it does not differ between taken Stress rehevmg PWHT is appitcabie to TMCP steels JISconvenuonal steels and TMCP steels Therefore, the absolute (japan Industrial Standard) pressure vessel steels whose thickness

level of toughness after stram agmg is higher in TMCP steels is 38 mm and over have to be subjected to stress rehef PWHT m

than m convennonal steels because of the improved base metal order to reduce welding residual stresses and to improve matenal

toughness in TMCP steels propemes This is ruled on the premtse of the use of normalizedsteels and as-rolled steels For TMCP steels wth improved base

5 2 Flame healing characteristics metal toughness. PWHT may be performed only for residual

In steel fabrication, steels are formed to a desired shape by the

stress rehevmg In steel fabncatton, steels are formed to a desired shape by the

stress relieving

use of local plastic deformation or thermal contraction caused by Ftgure 26 shows the expenmental results of the effect of PWHT

hne or spot flame heating Microstructures at the locally heated on TMCP steels [90. 91]is seen that the welding residual

regions change dependtng on the heaung they are subjected to, stresses are satisfactorily removed by PWHT of 550*C withoutand thus their toughness and strength also vary The Japan significant reducuon of their strengths, mstead of PWHT of

Shipbuilding Qualtty Standard stipulates 600’C as the maximum 600*C This result suggests that the PWHT condition may be

allowable heaung temperature for flame heating followed by relaxed from 600’C to 550’C for TMCP steels The addition ofwater coohng However, heaung up to 900*C may be permitted if small contents of ntobtum is very effective through precipitationthe heated portton is air cooled to 500*C and then water cooled hardenmg agamst the strength reduction dunng PWHT But care

must be taken to prevent precipitanon embntthng of the steels

The mechanical properties were examined for normalized steels, caused by excessive addition of niobium [92]controlled rolled steels and TMCP steels when they are subjected tolocal heaung with varymg maximum heaung temperature and water 6. Weld metal properties in TMCP steel welding

coohng start temperature [89] All the steels were strengthened tosome extent and their toughness was reduced while the degree of 6.1. Toughness of weld metal

toughness degradation did not differ from steel to steel All thesteels were most degraded when heated to the dual phase tempera- The welding matenal of a Ti-B system is extensively used forture (between 600 and 700*C) and then air cooled, because of the high heat input weldmg Hardenabihty of this welding matenal isresultant coarsened nucrostructures mixed with femte, bairate and controlled W1thm the appropnate range depending on weldingisland martensite Water

quenchingfrom these

temperaturesmust be heat mputs to be employed, so that its rrucrostructure mostly con-

avoided and the rule of air coohng to 500*C should be kept-TMCP sists of fine acicular femte (IAF) nucleated from Ti-oxtdes while-

steels are also supenor to conventional steels 10 flame heaung char- impeding the precipitation of grain boundary femte (GBF) and

actensucs because of their improved base metal toughness upper baitute (Bu) to a minimum The appropnate amount of free


13/16


- J As Welded tl 5501rx I2h

600’t Ix 2h2h

(’C) 260 -l4Mn- i6Mo-Ti-B

f

(t)it&horbar;&horbar;&horbar;K 1 -0 - 14Mn-16Mo-T!-8 637)- -20-1. .t.}20 300 ppm Oxygen

637)--20

250 O--- --40 a > 240- 5) 4 6kJ/mm4 6 k JTS(MPa) /mm/

:-

-

·___Sll._____________·1 FL _ -60 E240 - (545) () T S (M Pa) / _-40

200-

O---lL +2mm 9 0

> 200 -lr-------------

- -80


14/16

-

388 TMCP STEELS AND THEIR WELDING

,are generally ,u,,ceptible to ’ohdlflcallon cracking However 2 A J De Xrdo Proc Int Ssmp on Awelerored Cnnlme ofweld metals woh e1(ce&dquo;lvely reduced carbon are also vulnerable Rolled Steel (Edued b G E Ruddle). CIM. winnipeg Canada10 ,ohdlficallon cracking 1951[ 1988

Solidification cracking is more likely as the weldmg velocity 3 Monkawa. B10nyama ltoh J Jpn Weld Soc. Vol 55 No 2increase, Therefore, a root weld in pipe girthh weldmg is very pp 83-90. 1986 (in Japanese)susceptible to solidificauon cracking because root welds are madein a vertical-down manner at very high speed Figure 28 is one 4 Japan Iron & Steel Institute Properties of TMCP Steelf forexample of a solidification crack in the low carbon region In Preuure Vessels, 1986 (in Japanese)weldmg of the ultra-low carbon TMCP steels for line-pipe useThis type of solidification crackmg occurs in V-groove cellulose 5 Terashima. Furuklml J Jpn Weld Soc Vol 55 No 7electrode weldmg when the weld metal carbon is less than 005% pp 411-418. 1986 (in Japanese)>[96] The reduction of carbon content in a steel to a level less than0 02% leads to unexpected coarsening of austenite grains in 6 Ohashi, Mochituki. Yamaguchi Settetsu Kenkw, Vol 334,HAZs Therefore, high grade line-pipe steels contam carbon at pp 17-28, 1989 (in Japanese)0 05% or thereabouts for concurrent consideration of the avoid-

ance of solidification cracking and the HAZ hardness hmltallon 7 Tsuchida. Yamaba, Yamaguchi CAMP. Vol 2. pp 1724-1727,1989 fin Japanese)

t ’ ’ ’ ’1. ;.. 8 Matuszaki. Saito, Shiga CAMP, Vol 2, pp 1728-1731. 1989

j/ (in Japanese)

e . 9 Takechi Tetsu-to-Hagane, Vol 68, No 9, pp 1244-1255 1982(in Japanese)10 Ikenaga. Taklta, Mizui CAMP, Vol 2. p 759, 1989 (in

‘

s Japanese)

11V F Zackay. E R Parker, D Fahr, R Bush Trans Am SocMet , Vol 60. pp 252-259, 1967

12 Tsukaya. Kamel, Sakai CAMP, Vol 1, No 3. p 945, 1988.

2mm 13 Yamamoto. Kobayashi, Hondda CAMP. Vol 2. No 5.Fig 28 Solidification crack In pipe girth root weld of very low carbon p 1732, 1989 (in Japanese)

steel 196114 Ida, Takeshima Fujimoto CAMP, Vol I No 5, p 15091988 (in Japanese)

15 Kageyama, Sugmo, Fukuda Seitetsu Kenfsu, Vol 3297. Conclusion pp 2-14. 1988 Un Japanese)

aIt has been possible to manufacture high quality of steels by a 16 Wada, Fukuda Tetsu-to-Hagane, Vol 73. No 9.

O precise control of chemical composiuons together with employ- pp 1162-1169, 1989 On Japanese>ment of TMCP TMCP steels are not permitted to be subjected toheat treatments in which the temperature employed exceeds the 17 Japan Shipbuilding Research Association Report No 100.steel transformation temperature TMCP steels are supenor to Research on HT50 High Strength Steels by New Process, 1985conventional steels with respect to strain aging embnttlement, (in Japanese)flame

heatingembnulement,

toughnessafter PWHT, HAZ hard-

ening, cold cracking susceptibility, and HAZ toughness 18 N Yunoka, M Okumura, T Kasuya Metal Const, Vol 19,However, there are some problems concerning HAZ softening, No 4, pp 217R-223R, 1987weld metal toughness and solidification cracking when weldingof TMCP steels 19 N Yunoka Advances In Welding MetallurgB, pp 51-64,

AWS-JWS-JWES, 1990

Acknowledgements20 N Smith, B I Bagnall Metal Const , Vol I, No 2,

The author is grateful for Dr S Aihara, Dr Y Hagtwara, Dr H pp 17-23, 1969Tamehiro, Mr T Saito, Mr Y Hom and Dr M Okumura of theSteel Research Laboratones of Nippon Steel for their expert 21 E J Ridal Metal Const , Vol 3, No 11, pp 413-417, 1972advice and assistance He is also thankful to Prof De Meester of :Universitd Cathohque de Louvain and Dr C Shiga of Kawasala 22 D McKeown, P Judson, R L Apps Metal Const , Vol 15,Steel for their advice on the revision of this manuscnpt. No II, pp 667-673, 1983 ¡

References 23 P Hart Metal Const , Vol 18, No 10, pp 610-616,1986

1 G F Melloy, C W Roe and R D Romenl: Industnal Heating. 24 Okumura, Kasuya, Yunoka: Quarterly J Jpn. Weld Soc , VolVol. 5, p. 896, 1967 6, No. 1, pp 144-150, 1988 (in Japanese)


15/16


16/16

_390 TMCP STEELS AND THEIR WELDING

74 Njkantsh. Komiru Fukada (3«

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Tmcp Steels and Their Welding 12342