calculation of steel weldability and weld metal property

7/28/2019 Calculation of Steel Weldability and Weld Metal Property

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Updated, May 2007The calculation accuracy of welding cooling time was improved.The boundary condition at the plate surface in SAW was changed to the adiabatic condition.(The thermal reflection rate was changed from 0.9 to 1.0)Any input exceeding the limit is rejected and replaced by the limit value.

Master code by N. Y.

1. Formulae of carbon equivalent and steel transition temperatureTransformation temperatures (oC)Ac3=937.2-436.5C+56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr+38.1Mo

+124.8V+136.3Ti-19.1Nb+198.4Al+3315B

Ac1=750.8-26.6C+17.6Si-11.6Mn-22.9Cu-23Ni+24.1Cr+22.5Mo

-39.7V-5.7Ti+232.4Nb-169.4Al-894.7B

Ms=521-353C-22Si-24.3Mn-7.7Cu-17.3Ni-17.7Cr-25.8Mo

Carbon equivalents(wt%)The unit of chemical composition is wt%.

CE(IIW)=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5

CE(WES)=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14

Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B

CEn=C+f(C){Si/24+Mn/6+Cu/15+Ni/20+(Cr+Mo+Nb+V)/5}

where, f(C)=0.75+0.25tanh{20(C-0.12)}

.2. Equation of welding thermal historyThe calculation is based on the following equation in which the effects offinite plate thickness and heat transfer on the plate surfaces are consideredon the original Rosenthal equation.

Calculation menu of steel weldability and weld metal property1. Carbon equivalents andtransformation temperature

1. Remarks 4. Minimum necessarypreheat temperature1.Remarks

2. Calculation 2. Calculation

2. Thermal history and weldingcooling time in arc welding 1. Remarks 5. Weld metal tensilestrength 1. Remarks2. Calculation 2. Calculation3. HAZ maximum hardness 1. Remarks 6. Charpy transition temp. ofall-weld-metal

1. Remarks

2. Calculation 2. Calculation

Page 1 of 7Determination of necessary minimum preheating temperature

16/3/2010http://homepage3.nifty.com/yurioka/exp.html


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where,

T: temperature (oC)

Tph: preheat & interpass temperature (oC)

: ambient temperature (oC)

Tw: temperature increase due to a moving point heat source(oC)

x: coordinate in the welding direction (cm)

z: coordinate in the plate thickness direction (cm)

y: coordinate in the direction perpendicular to the welding direction (cm)

w: moving coordinate in the welding direction, w = x - v*t

v: welding velocity (cm/s)

t: time elapsed after the point heat source passed the static coordinate

origin (x = y = z = 0), (s)

R:

Rn:

Rn':

Qp: energy of heat source (cal/s)

h: plate thickness (cm)

: arc thermal efficiency, = 1.0 (SAW), 0.80 (SMAW, GMAW), 0.60 (GTAW)

: heat transfer coefficient at the plate surface = 0.0005cm/s (SAW)= 0.0020cm/s (SMAW, GMAW, GTAW)

: heat transfer coefficient at the surface except the weld part= 0.0020cm/s

r: heat reflection rate at the plate surface = 1.00 (SAW), = 0.80 (SMAW, GMAW, GTAW)

: thermal conductivity = 0.06 + 0.000012 * HI (cal/cm s)

: thermal diffusively = 0.042 + 0.000016 * HI (cm cm/s)

E: arc energy, E = 60 * A * V / v (J/cm)

HI: heat input, HI = * E (J/cm)

A: welding current (A)

V: welding voltage (V)

The above heat conduction equatiion is mathematically incorrect since the heat reflction rate, r is contained."r" was introduced so that the prediction could be more precisely made.The accuracy of the prediction is shown in

N. Yurioka. "Prediction of weld metal strength", IIW Doc. IX-2058-03




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3. Equation of HAZ maximum hardnessHAZ maximum hardness is estimated by equations described in the following paper:

N. Yurioka et al., "Prediciton of HAZ hardness of ferritic steels", Metal Construction, vol19 (1987), p.217R

where,

t8/5: welding cooling time between 800 and 500oC (s)The unit of chemical compostion is wt%.

f(B): an increase in HAZ hardenability due to boron, (C


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4. Prediction of minimum necessary preheat temperatureThe minimum necessary preheat temperature is predicted based on a method described inthe following paper:

N. Yurioka and T. Kasuya: "A chart method to determine necessary preheat in steel welding"

Welding in the World, vol. 35 (1995), p. 327-334

The validity of this method is compared with the British Standard and American Welding Societymethod:

N. Yurioka: "Comparison of preheat predictive methods"

Welding in the World, vol. 48 (2004), p. 21-27

The objective of preheating is to effuse diffusible hydrogen out of welds to prevent hydrogen-assistedcold cracking. The occurrence of cold cracking is influenced by the following factors:

1) Chemical composition of steel;2) Plate thickness or wall thickness;3) Weld metal diffusible hydrogen content4) Welding heat input5) Welding residual stresses or weld metal yield strength6) Weld joint restraint7) Notch concentration factor at weld toe and weld root or groove shape8) Weld pass number9) Preheating method (Heating rate, heating width)

10) Ambient temperature11) Immediate postheating

The present predictive method considers most of the factors above mentioned.

1) Chemical composition of steel

The following carbon equivalent has been long used as an index representing the susceptibility to

cold cracking. or weldability.

CE(IIW) = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 [wt%]

This carbon equivalent satisfactorily evaluates weldability whose carbon content is higher than 0.12%.Modern low alloy steel is mostly of a carbon reduced type (C


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Rutile electrode : 30ml/100g;Cellulosic electrode : 60ml/100g;Low hydrogen electrode : 5 - 8ml/100gUltra low hydrogen electrode : 2 - 5ml/100g

TIG, Solid wire GMAW : 2ml/100g;Flux cored wire GMAW : 6 - 10ml/100gSMAW : 2 - 8ml/100g

4) Welding heat input

With increasing heat input, the cooling rate decreases (the welding cooling time between 800 and 500oC, t8/5and welding cooling time to 100oC, t100 increases) and thus, a risk of the occurrence of cold cracking is reduced. Roughly speaking, cold crackingis a matter of concern only when heat input is not higher than 3kJ/mm.

5) Welding residual stresses or weld metal yield strength

Welding residual stresses are one of the important factors in cold cracking. The welding residual stresses oftenattain the yield strength of weld metal. Hydrogen-assisted cold cracking is more likely occur in welding ofhigh strength steel with using high strength welding materials.

6) Weld joint restraint

The weld joint restraint affects the cold cracking occurrence in one-pass welding.. In multi-pass welding, theoint restraint influences cold cracking to much lesser extent because a joint has been restrained after root-pass welding. Very low restraint may cause bending distortion leading to high bending stresses in weld root.As a result, root cracking may be caused.The present predictive method does not consider the effect of joint restraint.

7) Notch concentration factor at weld toe and weld root or groove shape

Cold cracking is more likely to occur at the root pass in the first side of double bevel groove (K groove, Xgroove) because of a high notch concentration factor at the root. However, the root weld of the first side isgenerally gouged before the start of second side welding. In welding with V groove and single-bevel groove,a notch concentration factor at the root is far less than that in double bevel groove welding. Therefore,the present predictive method does not consider the effect of a notch concentration factor.

In partial penetration welding with Y groove or single bevel groove, it is difficult to detect root cracking.Therefore, it is desired to employ the preheat temperature for repair welding.

8) The number of weld passes

In muti-pass welding, a root pass is reheated by subsequent passes so that residual stresses as well ashydrogen in the root bead are reduced. As a result, root cracking is less likely to occur in multi-pass weldingthan in one-pass welding..

This predictive method firstly gives the preheat temperature necessary to avoid root cracking in y-grooverestraint testing in which a one-pass short bead is deposited with high restraint as well as high notchconcentrations. This testing is so sever that much higher preheat is required than in normal welding practices.For normal welding, this predictive method gives preheating temperatures much lower than that for y-groove testing.. For instance, the necessarypreheating temperature for normal welding is 75oC less than that for y-groove testing when YP380MPa class steel is welded.

9) Welding residual stress

This predictive method considers the effect of welding residual stresses. The maximum welding residual stress is considered to be close to theyield strength of the weld metal. For higher strength steel, HAZ toe cracking, HAZ under bead cracking and weld metal transverse cracking aremore likely other than root cracking. As mentioned above, the necessary preheat can be decreased from that obtained by y-groove testing.However, the amount of this temperature reduction decreases as the steel strength increases (the weld metal strength also increases and weldingresidual stress increases as well). For instance, the temperature reduction is 75oC for YP360 steel and 0oC for YP700 steel.

In this predictive method, the yield strength of weld metal has to be input. When it is unknown, the specified minimum yield strength of the steelmay be input.

10) Preheating method

The objective of preheating is to enhance the hydrogen evolution from a weld. The effect of preheating increases as the width of preheatingincreases and the heating rate of preheating decrease. The preheating width over 200mm each side of the groove is desired. The preheatingtemperature has to be increased in the case of rapid preheating and narrow local preheating.

11) Ambient temperature

The occurrence of cold cracking is significantly affected by the ambient temperature. The cracking is more likely at the lower temperatures. As forthe determination of preheat at lower ambient temperatures, the following paper should be referred to.

T. Kasuya and N. Yuiroka: "Determination of necessary preheat temperature to avoid cold cracking under various ambient temperatures", ISIJInternational, vol. 35 (1995), No.10, p.1183-1189

12) Immediate post heating

Post heating immediately after welding is very effective for the hydrogen evolution. When the predicted necessary preheating temperature isexcessively high, immediate post heating should be employed so that the necessary preheating temperature could be reduced.




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150 oC for 95 hrs, or 200 oC for 29 hrs, or 250 oC for 12 hrs, or 300 oC for 2 hrs.

5. Prediction of weld metal tensile strengthThis predictive method is base on the following paper.

N. Yruioka: Perdition of weld metal strength, IIW Doc. IX-2058-03

This method first predicts the weld metal hardness, Hv from weld metal chemical composition (C, Si, Mn, Cu, Ni, Cr, Mo, V, Nb, Ti[wt%]) and thewelding cooling time (t85[s]).

Hv = (HM + HB)/2 - (HM-HB) arctan(x)/2.2

x = 4 log(t85/tM)/log(tB/tM) - 2

HM = 884C + 294

tM (s)= exp(10.6CEI - 4.8)

CEI (wt%)= C + Si/24 + Mn/(2.88(1 +Mn)) + Ni/30 + Cr/16 + Mo/8

HB = 145 + 130 tanh(2.65CEII - 0.69)

CEII(wt%) = C + Si/24 + Mn/(2.16(1 + Mn)) + Cu/10 + Ni/45 + Cr/10 + Mo/5 +2V + 2.2Nb/(1 + 5Nb) + Ti/10

tB (s)= exp(6.2CEIII + 0.74)

CEIII (wt%)= C + Mn/(1.68(1 + Mn)) + Ni/15 + Cr/10 + Mo/8

Then, Hv thus obtained is converted to the weld metal tensile strength, TS.

TS(MPa) = 3.0Hv + 22.3

6. Prediction of weld metal toughnessThis perdition is preformed by a neural network analysis of the database of low alloy weld metal from the University of Cambridge(http://www.msm.cam.ac.uk/map/data/materials/)

The weld metal in the database is all-weld-metal obtained under a constant welding condition of arc energy of 1kJ/mm (heat input of 0.8kJ/mm),interpass temperature of 200oC and plate thickness of 20mm. The toughness is given by the transition temperature for the Charpy impact value of28J.

A software developed by D. J. C. Mackay at the University of Cambridge was used for the neural network analysis. The prediction gives the degreeof the prediction error. When the difference between the max and min predicted values is over 30oC, the prediction may be unreliable.

The following figure shows the relation between the estimation (vertical axis) and the database (horizontal axis).




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Ver. 1.5 - Updated, June 2008


calculation of steel weldability and weld metal property

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