Stress-strain characteristics of the heat-affected zone in an HY-100 weldment as determined by microindentation testing

Stress-strain characteristics of the heat-affected zone in an HY-100 weldment as determined by microindentation testing

ELSEVIER Stress-Strain Characteristics of the Heat-Affected Zone in an HY-100Weldment as Determined by Microindentation Testing Joseph E Zarzour, Pau...

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ELSEVIER

Stress-Strain Characteristics of the Heat-Affected Zone in an HY-100Weldment as Determined by Microindentation Testing Joseph E Zarzour, Paul J. Konkol, and Hao Dong Concurrent

Technologies

Corporation,

Johnstown,

PA

Microindentation hardness tests were carried true strain data obtained by “automated ball useful information for the various regions of tional mechanical testing is difficult. Detailed an effort to correlate microstructural features

out on HY-100 weldments. The true stressindentation” on these weldments provided the heat-affected zone (HAZ), where tradimicrostructural analyses were performed in with mechanical properties established for

different regions of the HAZ. The results provide a relation between grain size and corresponding stress-strain data across the HAZ. 0 Elseuier Science Inc., 1996

INTRODUCTION

may contain undissolved carbides owing to the low peak temperature. In region 4, there has been some grain growth and homogenization. Region 5 was subjected to temperatures close to melting, with substantial grain growth. In addition to chemical composition and microstructure, grain size and size distribution also affect fracture toughness. Previous work shows that the toughness transition temperature is lowered by reducing the prior-austenite grain size [2, 31. Therefore, improved control of grain size in the HAZ is very important for optimizing the fracture toughness of weldments. The effect of the interaction between thermal history, chemical composition, and welding parameters on HAZ toughness is complex and strongly dependent on grain size. HAZ grain size can be reduced by several methods. One method is to reduce the weld energy input, which will reduce the width of the HAZ and thus reduce the volume of the high peak temperature region (region 5). In multipass welds, region 5 may be refined by reheating to a lower peak temperature (region 3 or 4) by the heat from a subsequent weld pass, depending on energy input and weld sequence. Another method is to produce a steel with

The heat-affected zone (HAZ) contains a spectrum of microstructural features that can display a wide range of material properties, including fracture toughness values. These variations in microstructure are directly related to the peak temperature and rate of cooling in the welding process. Regions that are close to the fusion line are subjected to temperatures close to or at melting, whereas remote regions may be only moderately elevated in temperature. Consequently, the HAZ regions are subjected to a continuous temperature gradient. The various regions of the HAZ in a ferritic steel are shown schematically in Fig. 1 [l]. The unaffected base metal is identified as region 0. Region 1 was heated to below the lower critical temperature for the start of austenite transformation (Al) and subjected to additional tempering of the base metal in quenched-and-tempered steels. Region 2 was heated between the A1 and the upper critical temperature for complete austenite transformation (AS) and contains a mixture of newly transformed austenite grains and overtempered base metal. Region 3 was completely transformed to austenite with little or no grain growth and 195 MATERIALS CHARACTERIZATION 37195-209 (1996) 0 Elsevier Science Inc., 1996 655 Avenue of the Americas, New York, NY 10010

1044-5803/96/$15.00 PII SlO44-5803(96)00128-3

J. F. Zarzour et al.

196

microindentation, a nondestructive technique, a profile of the true stress-true strain curve can be determined for all regions of the HAZ. Second, using a statistically based scheme, an evaluation of the prior austenite grain sizes can be obtained for each HAZ zone. These results are integrated to obtain a correlation between microscopic (grain size) and macroscopic (stress-strain) material properties. The implication of this relation is that, because the lowest fracture toughness zone in the HAZ is not known in advance, a notched specimen can be prepared with the crack and crack-tip opening displacement dimensions precisely located over the zone of interest. In addition, because the notch-tip radius of the fracture toughness specimen should not be less than five grain diameters, the notch-tip radius can be predetermined. Without prior knowledge of the location and grain size of the various HAZ regions, it is very difficult to conduct a “true” HAZ fracture toughness test because the notch-tip in a small material volume inevitably samples other HAZ material zones. No satisfactory method is available for determining the actual fracture toughness of a single HAZ region. Although a weld simulation (Gleeble) method for producing the desired microstructure in a specimen by duplicating the thermal history exists [8], the volume of the microstructure of interest and the material constraint are very different from those of the actual weldment. Moreover, Gleeble weld HAZ specimens are limited in size and may not manifest the effect of residual stresses or meet the minimum size requirements for a valid fracture toughness test.

TEMPERATURE

FIG. 1. Relations among peak temperatures, distance from weld interfaces, and the iron-carbon phase diagram [l].

carbides, nitrides, or nonmetallic inclusions that remain undissolved in regions 4 and 5 and thus act as barriers to austenite grain growth. However, previous studies [4-6] suggested that this technique may adversely affect other material properties, including the ductility and strain hardening of the base metal. Two HAZ regions with low fracture toughness are typically found in 1. the coarse-grain zone (region 5) adjacent to the weld interface in the last weld pass during multipass welding, 2. portions of underlying region 5 not subjected to reheating by subsequent weld passes and thus not benefitting from grain refinement. Depending on steel composition and weld cooling rate, low fracture toughness has also been observed in the region heated between Al and A3 (region 2) of the HAZ [7]. Two important sets of information can be obtained from the experimental results of the present study. First, through the use of Table 1

PLATE

PROPERTIES

The base material is 51mm thick HY-100 steel plate produced in accordance with

Chemical Composition of 51mm Thick HY-100 Base Plate

C

Mn

P

S

Si

CT

Ni

MO

Ti

CU

V

0.16

0.35

0.010

0.006

0.23

1.61

2.87

0.56

0.010

0.08

0.01

HAZ Stress-Strain Table 2

Characteristics

197

in HY-100

Tensile Properties of HY-100 Base Plate Tensile 0.2% YS

Specimen orientation to rolling direction

Transverse Longitudinal

Compressive 0.2% YS

(MPal

(MPa)

740 746

763 766

MIL-S-16216K (SH) [9]. The microstructure is tempered martensite with an average prior austenite grain size of about 20km. The chemical composition and mechanical properties, including tensile and compressive yield strengths (Ys), are given in Tables 1 and 2, respectively. Small differences between the values of compressive and tensile yield strengths were observed for specimens for which extractions were both parallel and perpendicular to the rolling direction.

WELDMENT

PREPARATION

MICROSTRUCTURAL CHARACTERIZATION

The welding parameters used to prepare the weldment are summarized in Table 3. The weld interface was straight (Fig. 2) and parallel to the edge of the plate (bevel angle equal to zero), and the root opening was 76mm wide. The resulting HAZ zones were about 7mm wide (see Fig. 4).

SPECIMEN

PREPARATION

A metallographic specimen (Fig. 3) was machined from the center of the weldment normal to the welding direction. The specimen was polished to a 0.05Fm finish fol-

Table 3

lowed by etching with a 2% nital solution to reveal the HAZ microstructure. Figure 3 shows the weld passes and the indentation sites across the HAZ. Square grids lmm by lmm were placed on the etched surface of the sample to identify each grid location for grain size and microstructure. Microstructural analyses were conducted along a line containing Grids O-6 (Fig. 4), sampling the unaffected base metal (Grid 0), the HAZ (Grids l-5), and the bulk weld metal (Grid 6). Grids 1 to 5 correspond to the HAZ regions shown in Fig. 1. The grid line of interest was chosen so that it passed through a HAZ that was not significantly heated or tempered by subsequent adjacent weld passes. Other grid locations (Grids 7-9) in the weld metal and base metal, which are schematically shown in Fig. 4, were used to determine if any variations in material behavior existed.

Gas-Metal-Arc Welding (GMAW) Parameters

Voltage (volts)

Current

Travel speed

Energy input

(amps)

(mm/d

&]/mm)

Number of passes

29

329

4.28

2.2

30

Weldment deposited by using GMAW spray transfer mode in flat position with 95%Ar + 5%CO2 shielding gas and 1.6mm diameter type MilLlOOs electrode per Mil-E-23765/2E.

Photomicrographs were taken in the central area and four corners of each selected grid at an appropriate magnification for proper grain size measurements. Prior austenite grain sizes were measured directly from each photomicrograph, using the American Society for Testing Materials intercept method. Measurements of the prior austenite grain sizes are shown in Figs. 5-l 1. There are significant changes in grain sizes across the HAZ regions. From the base material-HAZ interface to the HAZ-weld interface, grain size decreases to a minimum value in the central area of Grid 3 (finegrain HAZ, Fig. 8) and then increases to a maximum value at the center area of Grid 5 (coarse-grain HAZ, Fig. 10). The abrupt changes in grain size are accompanied by a change in the microstructure from the base metal across the HAZ to the weld metal. Only the microstructure of the central area of each grid is presented in this paper. The base metal (Grid 0) basically shows a microstructure of tempered martensite with some ferrite (Fig. 5). The subcritical HAZ,

J. F. Zarzour et al.

198

t 51mm

i

(4

layer 8 layer / layer 3 layer 1 layer 2

Microindentation traverse

layer 5 layer 6

(b) FIG. 2. Schematic showing (a) base-plate preparation and first two weld layers and (b) complete weldment of eight layers following removal of base ligament after deposition of layers 1 and 2. Microindentation traverse location also is shown.

Grid 1 (Fig. 6), which underwent additional tempering from the weld pass, shows a microstructure similar to that of the base metal. The intercritical HAZ, Grid 2 (Fig. 7),

a

shows tensite, heated higher

a fine-grain microstructure of marferrite, and carbides. This zone was to a temperature equal to or just than the temperature for partially

b

FIG. 3. Actual specimen featuring (a) various weld passes and (b) exact location of the HAZ indentation

sites

HAZ Stress-Strain Characteristics 7mm

in HY-100 7mm

199 5, located (Fig. 13).

adjacent

to the weld

INDENTATION

FIG. 4. Schematic of the locations of selected grids prepared for the indentation tests across the weldment.

transforming to austenite (Ai temperature). The fine grain size is the result of partial formation of new austenite grains. The fine-grain HAZ, Grid 3 (Fig. B), shows a microstructure of martensite/bainite, ferrite, and carbides, with the finest grain sizes compared with other locations in the HAZ. This zone was subjected to a temperature slightly higher than the temperature for fully transforming to austenite (A3 temperature). The medium-grain HAZ, Grid 4 (Fig. 9), shows a microstructure of martensite/bainite, ferrite, and carbides, with a slightly coarser grain size. This zone was heated to a temperature significantly above As, at which austenite grain growth occurred. The coarse-grain HAZ, Grid 5 (Fig. lo), which is closest to the weld interface, shows a microstructure of coarse martensite/bainite packets and ferrite and large prior austenite grains. This zone was heated to a temperature close to the temperature of partial melting. Grid 6 (Fig. ll), shows a microstructure typical of weld metal. Owing to its lower hardenability, the weld metal microstructure is primarily ferritic. A continuous profile of these microstructural variations from the base metal through the HAZ to the weld metal is shown in Fig. 12. In summary, the finest grain sizes were observed in Grid 3, which is located in the central area of the HAZ (Fig. B), and the coarsest grain sizes were observed in Grid

interface

TESTS

After the microstructural analysis of grids corresponding to the base metal, weld metal, and various HAZ regions with various prior austenite grain sizes, microindentation tests were performed over the central location of the same grids. The indentation tests, known as automated ball indentation (ABI), were performed at Advanced Technology Corporation.* Detailed information on this technique is given in References 10-12. This technique utilizes electromechanically driven spherical tungsten carbide indenters ranging in diameter from 0.25 to 1.57mm and attached to a portable stress-strain microprobe system. The choice of the indenter diameter is dictated by the average grain size of the material to be evaluated. The minimum diameter of the indenter should be approximately five times the size of an average grain. The basic mechanism of the ABI test, which enables the evaluation of true stress-true strain curves, is the use of multiple indentation cycles (at the same indentation location) on a polished flat metallic surface, using a spherical indenter. Upon loading, unloading, and reloading sequences, quantities including indentation load, penetration speed (strain rate), and penetration depth (displacement) are measured and recorded with a PC-based data acquisition system. Initial and final allowable penetration depths are chosen on the basis of maximum load or maximum total strain value. The yield stress is determined from knowledge of several parameters such as the total penetration depth, diameter of the indenter, applied indentation load, and one material constant and one material test parameter at each loading cycle. The load-displacement data from each indentation se-

*Advanced Technology Corporation, Drive, Oak Ridge, TN 38730-7665

115 Clemson

200

J. F. Zarzour et al.

800 600

-

(J = 1062.7 cp, (“.046988)

400

-

0. yle,d= 744.66

200

-

0 0

(h4Pa)

I

I

0.05

0.1

True Plastic (a)

0.15

Strain

(b) FIG. 5. Grain size (20.7p.m) and corresponding

quence are fitted to a first-degree polynomial and the fit extrapolated to obtain the displacement corresponding to zero load. The displacement values from each indentation sequence are used, along with the yield strength, to derive the true stress-true plastic strain curve of the indented material.

true stress-true

strain data for Grid 0 (base metal).

RESULTS

Sixteen indentation tests were conducted, using a 0.5mm diameter tungsten carbide indenter. For all indentations, the indenter head was selected to be large enough to cover five or more grains of the largest av-

HAZ Stress-Strain

Characteristics

1200

in HY-100

201

I

1000

-

600

-

I

0

0

(3 = 1055.7 E (0.076403) P’

400

-

200

0

0

0.y,e,d= 648.82 (MPa)

I

I

0.05

0.1

0.15

True Plastic Strain (a)

(b) FIG. 6. Grain size (20.7km) and corresponding

erage to fit speed dition were

grain size (23pm) and small enough in a lmm square grid. An indenter of O.O0381mm/s was selected. In ad, six indentation and six tensile tests conducted on HY-80, HY-100, and

true stress-true

strain data for Grid 1 (HAZ)

HY-130 plates (all base metal) to calibl rate the material constants. The indentation tl were performed up to about 15% of the true plastic strain. The results are repor ted for grids that were characterized for gi -ain

J. F. Zarzour et al.

202

1200 1000

I

I

-

0

z

n

0 “000

E co

-

5t2

600

2

400

-

200

-

t-’

(3 = 1076 E W’745’W PI

0

CJ

yield

=

661.23 @IPa)

I

0

I

0.05

0.1

0.15

True Plastic Strain (a)

(b) FIG. 7. Grain size (11.4pm) and corresponding

size, (Grids O-9, Fig. 4). In each of Figs. 5-11, the true stress-true plastic strain is shown with the corresponding microstructure. As explained earlier, the reported grain size value is the average at the location, which includes the four corners and the center of

true stress-true

strain data for Grid 2 (HAZ).

each grid. Also shown is the expression of the power-law fit and the yield strength. The unaffected base metal (Grid 0, Fig. 5) has low hardening behavior and average yield strength of 745MPa, which is in close agreement with previous mechanical tests

HAZ Stress-Strain

Characteristics

in HY-100

203

800 o

=

1299

(0.055146)

E

PI

600 0. y,e,d =

916.34

(MPa)

I

I

0.1

0.05 Try

FIG. 8. Grain size (6.2Fm) and corresponding

[13] and data in Table 2. The average grain sizes were about 21k.m. A large gradient of yield strength was found within the various HAZ zones: from 649MPa associated with an average grain size of 20.7km (Fig.

a

plastic

Strain

true stress -true strain data for Grid 3 (HAZ).

6) to 916MPa associated with an average grain size of about 6.2Cl.m (Fig. 8). The weld metal (Grid 6, Fig. ll), which has a columnar microstructure, had a yield strength of about 600MPa, which is slightly lower than

204

1. F. Zarzour et al.

o

=

1239.3

E

WJ60734)

PI

600

<3yield= 837.05 (MPa)

400

-

0

.

I

0

0.05

I

0.1

0.15

True Plastic Strain (a)

(b) FIG. 9. Grain size (9.1km) and corresponding

the iaverage yield strength (668MPa) typicallp r obtained from an all-weld-metal tentest specimen. A second set of tests was conducted to investi igate any variations in the true stresstrue strain data of the weld metal at three

true stress-true

strain data for Grid 4 (HAZ).

different locations in the specimen. As shown schematically in Fig. 4, grids were equidistant from each other in the weld metal. The material stress-strain behavior is expected to have some variations because different locations in the weld are subjected to differ-

HAZ Stress-Strain

Characteristics

400

-

200

-

in HY-100

205

(3y,e,d= 828.1 (MPa)

I

0 0

I

0.05

0.1

Try

rlastic

0.15

Strain

a

(b) FIG. 10. Grain size (22.6km)

and corresponding

ent thermal histories during welding. Indeed, the yield strength varied from one grid location to another. For example, from Grid 6 to Grid 8, the yield strength increased by about 10%. An overall variation in the yield strength

true stress-true

strain data for Grid 5 (HAZ).

with respect to position (from Grid 0 to Grid 9) is shown in Fig. 14. As discussed earlier, there is a steep gradient in yield strength across the HAZ, which is associated with variations in microstructure and grain size and with the distribution of the

J. F. Zarzour et al.

206

800 600 (3

=

929.7

(0.068687)

E

P’

0 yre,d= 599.17

(MPa)

0.05

0.1

True Plastic Strain (a)

FIG. 11. Typical weld metal microstructure

and corresponding

different HAZ constituents. This sharp difference in yield strength is also a strong indication of the variability in fracture toughness (yield strength is generally inversely proportional to the fracture toughness). True stress-true plastic strain curves for

true stress-true

strain data for Grit 36

all HAZ regions are plotted together in Fig. 15 to show the amount of variability in yield strength and strain hardening. It is important to note that the gradient in both yield strength and strain hardening is so large that a unique stress-strain curve

HAZ Stress-Strain

Characteristics

in HY-100

207

l



(Base) FIG. 12. Microstructural

montage showing variations

cannot be derived to represent “one HAZ material,” as is usually determined in finite element simulation of HAZ-related studies. Grain size and yield strength curves as a function of grid location are shown in Fig. 16. As expected, there is an inverse relation between the grain size and the yield strength. However, the inverse relation is not strictly linear. As shown in Fig. 17, the yield strength is also dependent on microstructure and prior thermal history. The overtempered region, Grid 1, has lower yield strength with no change in grain size because the peak temperature in this region was below A,; thus no new grains were formed. The intercritical re-

I

from base metal through the HAZ to the weld metal.

gion, Grid 2, which consists of a mixture of new untempered martensite and untransformed overtempered martensite, has a much finer grain size with no loss in yield strength. Only the completely reaustenitized and transformed zones-Grids 3, 4, and 5, which differ essentially only in grain size and not microstructur~xhibit the inverse correlation.

CONCLUSION

Automated ball microindentation, coupled with microstructural characterization of high-strength steel weldments, is a viable

/ --

I

z E .c

5 5 5 P j;

0

I

1

1

1

I

1

2

3

4

5

6

Grid Number

FIG. 13. Variation across the HAZ.

Base

HA2

I

II-

1000

24

0

(Weld)

OIAZ)

of grain size from the base metal

900

HP Weld

,

/

800 700 600 500

L

L 0

20

40

60

80

Distance (mm)

FIG. 14. Yield strength variations gas metal arc weldment.

! 100

120

across the HY-100

J. F. Zarzour et al.

208 1000

/

I

I

3 (tine)

1167 E

933.3

E D e! 0) J

700 466.7

F 600

233.3 0 0.1

0.05

0

0.15

True Plastic Strain

procedure for characterizing the behavior of HAZs. Although no additional evidence is available to substantiate the reliability of the indentation tests in the HAZ, a relation was observed between the microstructure, prior austenite grain size, and the stressstrain profile. In addition, for fracture toughness testing, this technique will permit proper positioning of the notch location and the size of the crack-tip radius relative to the average gram size. This investigation of the material properties of the HAZ led to a quantitative correspondence between the HAZ microstructure and true stress-true strain behavior, including the hardening and yield strength of each HAZ region. This work was conducted by the National Center for Excellence in Metalworking Technology,

24

1

2 Grid

I

I



-

900

-

650

-

600

$

-

750

s

-

700

z 2

-

650

3

4

5

1

5 g

,A 5

15

10

20

25

Size (pm)

FIG. 17. Effect of prior austenite grain size on yield strength in the HAZ of an HY-100 weldment. Numbers refer to grid locations.

operated by Concurrent Technologies Corporation, under contract to the U.S. Navy as part of the U.S. Navy Manufacturing Technology Program. The authors would like to acknowledge project management by Larry S. Knipple.

References 1. Welding Handbook. Vol. 1,8th ed., American Welding Society, pp. 110-125 (1987). 2. R. E. Dolby, Toughness of martensitic and martensitic-bainitic microstructures with particular reference to heat affected zones in welded low alloy steels. J. Iron Steel Inst. 210:857-865 (1972). 3. K. Minoda, K. Kohno, N. Katayama, S. Kaihara, and Y. Kaki, Crack opening displacement characteristics of welded joint on low temperature service steel for LPG storage tanks. Trans. Iron Steel Inst. Jpn. 22(12):942-951 (1982).

5. J. H. Chen and C. Yan, A comparison of toughness of C-Mn steel with different grain sizes. Metal Trans. A 23:2549-2556 (1992). 6. H. G. Pisarski and I’. L. Harrison, Welding Procedures for Heat-Affected Zone Fracture Toughness Assessment. Int. Conf. on Weld Failures, London, Vol. 21-24 (November 1993). 7. J. A. Davidson, I’. J. Konkol, and J. F. Sovak, Assessing fracture toughness and cracking susceptibility of steel weldments: a review. Welding Research Council Bulletin No. 345, July 1989.

600 6

Number

FIG. 16. Grain size and yield strength versus number across the HAZ of an HY-100 weldment.

1 (tempered, _

4. G. R. Wang, T. H. North, and K. G. Lewis, Microalloying additions and HAZ fracture toughness in HSLA Steels. Welding 1. (Research Supplement) 14s-22s (1990).

2

0

2 (intercritical)

Grain

FIG. 15. True stress-true plastic strain data for all HAZ zones of the HY-100 gas metal arc weld.

o-1

500

-

grid

8. R. D. Stout and W. D. Doty, Weldability of Steels. 3d ed., Welding Research Council, New York (1987).

HAZ

Stress-Strain

Characteristics

in HY-100

9. MIL-S-16216K(SH), Steel plate, alloy, structural, high yield strength (HY-80 and HY-100). 10. F. M. Haggag, Portable/In-Situ Stress-Strain Microprobe (SSM) System for Nondestructive Field Applications. Advanced Technology Corp., Oak Ridge, TN (1995). 11. F. M. Haggag, Application of flow properties microprobe to evaluate gradients in weldment properties. In International Trends in Welding Sciences and Technology, S. A. David and J. M. Vitek, eds., ASM, Metal Park, OH, pp. 843-849 (1993). 12. F. M. Haggag,

In-situ measurements

of mechani-

209 cal properties using novel automated ball indentation system. Reprint from Standard Technical Publication 1204, American Society for Testing and Materials (ASTM) (1993). 13. J. F. Zarzour and M. J. Kleinosky, Fracture Characteristics of HAZ-Double Edge Notched Weld ]oints with Mechanical Undermatching. Proc. Int. Symp. on High-Performance Steels for Structural Applications, Riad Asfahani, ed., ASM Materials Week ‘95, Cleveland OH, 197-203 (1995). Received May 1996; accepted September 1996.