TECHNICAL SERVICE REPORT 121

EVALUATION OF CRACKING AND CORROSION SENSITIVITY OF

USING INCONEL 112 ELECTRODES A CN-3MN CASTING / SMA BEAD-ON-PLATE WELDING

APRIL 1995

C.D. LUNDIN W. LIU

C.Y.P. QlAO K.K. KHAN

MATERIALS JOINING RESEARCH MATERIALS SCIENCE AND ENGINEERING

THE UNIVERSITY OF TENNESSEE, KNOXVILLE

STEEL FOUNDERS' SOCIETY OF AMERICA

EVALUATION OF CRACKING AND CORROSION SENSITIVITY OF A CN-3MN CASTING / SMA BEAD-ON-PLATE WELDING

USING INCONEL 112 ELECTRODES

INTRODUCTION

A section of a CN-3MN cast block 8" (long) X 2" (wide) X 2" (average

thickness) was welded with single pass (bead-on-plate) welds on each of 4

machined steps (Figure 1). The composition of CN-3MN is similar to AL-6XN. The

cast block was solution annealed (2250°F, 4 hours) before welding. The as-welded

surface of the block after dye penetrant testing is shown in Figure 2. The upper step

was defined as being 1/32" below the as-cast surface. The steps were machined in

1/8" increments. Liquid penetrant testing was conducted after machining and prior to

the deposition of the welds, and only one indication was found. Cracking in the HAZ

of all welds was clearly evident after welding. The composition of the casting (from

two analyses) and the typical for the weld metal is presented in Table 1.

Technique) on thin slices of material at the different depths. The chemical analysis

results are shown in Table 2. It may be noted from the results that a carbon enriched

zone is present at the cast surface, the thickness of which is greater than 0.09" but

less than 0.28".

Evaluation of the base metal and weld HAZ cracking mechanisms, unmixed

zone phenomenon, and corrosion behavior of the CN-3MN casting was carried out

in this study.

EVALUATION OF CRACKING MECHANISMS

1. Evaluation of Cracking Behavior of CN-3MN SMA BOP Welds with lnconel 112 Filler Metal

1-1. Dye Penetrant Testing

Four SMA weld passes are identified as A, B, C, and D, as indicated in Figure

2, at depths below the as-cast surface of 0.03", 0.16", 0.28", and 0.41" respectively. A

dye penetrant test was performed on the SMA bead-on-plate welds. The dye

penetrant test results are presented in Figure 3. A total of twenty six indications were

detected on the four welds as shown in Figure 3. Among them, six indications were

detected in the casting remote from the weld HAZ and twenty indications were

detected in the HAZs of the four welds as indicated in Table 3.

Although twenty cracks were observed in the weld HAZ by dye penetrant

testing, cracks could also be present in the casting below the cast surface and

remote from the welds. Metallogaphic and fractographic examinations were

conducted to determine if the cracks were formed during casting or were result of

we Idi ng .

1-2. Metallographic Evaluation

The specimens for metallographic evaluation were extracted from weld B in

the weld face and transverse orientations. They were polished to 0.05 um and

electrolytically etched with Glyceregia. Metallographic evaluation of cracking

behavior was performed on the polished and etched surfaces using both optical light

microscopy (OLM) and scanning electron microscopy (SEM). Chemical analysis

checks were carried out using EDS. Figure 4 shows a macrograph of weld B in the

weld face orientation. A coarse dendritic solidification structure is evident in the cast

base metal. Seven cracks were noted in this metallographic section (specimen size:

0.56" X 0.44"). The three cracks marked as a, b and c in Figure 4 were characterized

at a higher magnification (50X) . Cracks a and c in Figure 4 are shown in Figures 5

and 6, respectively

On the transverse cross-section of weld B, identical type of intergranular

cracks were observed in the HAZ as shown in Figure 7. It is to be noted that,

although the cracks shown in Figures 5 through 7 are adjacent to the fusion zone of

weld B, the location and extent (size) of the cracks strongly indicate that some of

these cracks were not initiated by welding. However, welding stresses may have

caused the cracks to extend beyond their original size. Also, some cracks (e.g. a1 -

a3 in Figure 5a) appear to exhibit more crack face separation than the other cracks.

In addition, some cracks ( e.g. a2 in Figure 5a and 5c) are back-filled with weld

metal.

Figures 8 and 9 show the SEM microstructural morphologies of the HAZ

cracks at different magnifications. Particles of various sizes are distributed along the

grain boundaries as well as in the grain matrix as shown in Figure 10. EDS

analyses were performed at the locations identified by arrows and letters in Figure

10. The major alloying and trace elements detected at these locations are

summarized in Table 4. It is to be noted that the EDS levels (%) of the elements

determined are not equivalent to these obtained from standard bulk chemical

analysis. The relative amounts, however, do provide a comparison between

locations and help to identify segregate levels which may be important to the

cracking observed. In addition, it should be pointed out that EDS chemical analysis

represents an average composition from the localized area (about 1 to 3 µ diameter

spherical volume) while the bulk chemical analysis (combustion analysis) represents

an average composition of a much larger sample.

Figure 11 shows the secondary electron image and back-scattered electron

image of the particles along grain boundaries at a magnification of 5000X.

Figure 12 shows the typical morphology of the liquated grain boundary in the

HAZ of the CN-3MN casting. EDS analysis was performed along this grain boundary

at the locations indicated by the arrows and letters. The major alloying and trace

elements detected at these locations are summarized in Table 5.

Particles distributed in the CN-3MN grain matrix in the HAZ were also

analyzed by the EDS technique at the locations indicated by arrows and letters in

Figure 13. The test results are presented in Figures 14 to 16.

According to the results obtained from SEM metallographic and the EDS

chemical examination, it is clear that the majority of particles distributed along the

grain boundaries and within the grain matrix are Cr rich sulfide, Mg, AI, and Ca rich

or Mg, AI, Ca, and Cr rich silicates, A Cu rich sulfide was also detected along the

grain boundaries. Mg, AI and Cu can lower the liquation temperature and degrade

cracking resistance. Therefore, the silicates found along grain boundaries can be

expected to exacerbate cracking tendency.

1-3. Fractographic Evaluation

The specimen for fractographic evaluation was extracted from the HAZ of weld

B. SEM fractographic evaluation was conducted on an opened crack surface.

Figure 17 shows the morphology of the crack surface at 33X. A smooth fracture

surface characteristic is evident, in addition to a visible solidification substructure.

The fracture surface morphologies at higher magnification are shown in Figures 18

and 19. It is to be noted that a thick oxide film (black color) is present on the fracture

surface. From the extent and the color of oxides on the fracture surface, it is

expected that the oxidation occurred during casting solidification and/or solution

annealing. Thus, this observation suggests that the cracks found in the weld HAZ

existed prior to welding. In order to verify this judgment, Varestraint testing was

undertaken and the test results are discussed below.

2. Evaluation of Cracking Behavior by Varestraint Testing

To define the weld induced HAZ crack surface morphology, the Varestraint

test method was used to create "fresh" cracks in the GTA weld HAZ for

metallographic and fractographic evaluations. A schematic drawing of the

Varestraint test device is shown in Figure 20. Two Varestraint test specimens, 1/8"

and 3/16" thick, were machined from the CN-3MN cast block. Photographs of the

Varestraint tested specimens are shown in Figure 21. The cracks in both the HAZ

and fusion zone are clearly visible.

2-1. Metallographic Evaluation

Metallographic specimens extracted from the Varestraint tested specimens

were polished to 0.05 um and eletrolytically etched with oxalic acid. It is to be

recognized that the HAZ cracking is more extensive compared to conventional

austenitic stainless steel Varestraint test data base in terms of both total and

maximum crack length. Typical HAZ cracks are presented in Figures 22 and 23.

EDS analysis was conducted on the particles along the grain boundaries at

locations U and V (see Figure 23) as shown in Figures 24 and 25. It is to be noted

that the particles on the grain boundaries are mainly silicates and sulfides in addition

to carbides and sigma phase. These results are consistent with the results obtained

on the fracture surface of the crack that existed in the HAZ of SMA BOP weld B.

2-2. Fractographic Evaluation

A HAZ crack in the Varestraint sample was opened to conduct fractographic

examination, Figures 26 and 27 show the fracture morphology of the HAZ crack

surface. Compared to the crack surface of the specimen extracted from the HAZ of

SMA weld B, a similar smooth fracture surface characteristic is observed on the HAZ

crack surface of the Varestraint specimen. Oxidation was also observed on the

fracture surface of Varestraint sample. However, the oxide of the crack surface

(especially near the fusion zone region) exhibits a light brown/blue color which is

different from that noted on the SMA BOP weld HAZ crack surfaces. This observation

on oxide character verifies that cracks pre-existed in the cast block prior to welding

and were oxidized during casting or solution treatment.

Various particle sizes were found on the HAZ fracture surface of the

Varestraint sample (see Figure 27). EDS analyses of these particles were performed

at the locations indicated in Figure 27. Several typical particles along with their EDS

spectra are exhibited in Figures 28 to 31. Predominantly, the particles on the fracture

surface are Mg and Cr rich silicates, Cr-sulfide and Cr-carbide.

From the test results obtained by metallographic and fractographic evaluation

using OLM and SEM with EDS techniques, it is evident that intergranular type cracks

were present in the cast block prior to welding. However, the weld process, served

to increase the crack length and crack opening. Particles significantly rich in Mg, AI,

and Ca were found along grain boundaries as well as in the CN-3MN grain matrix.

Cu was also detected along the grain boundaries in very fine particles. These

second phase particles can play an important role in reducing grain boundary

liquation temperature. Silicates found along grain boundaries also tend to

exacerbate cracking. In addition, the solidification cracking tendency during casting

can be indirectly revealed by using the Varestraint test since weld HAZ cracking and

casting solidification cracking have very similar cracking characterization, (both

cases are hot cracking).

The carbon gradient did not appear to influence the cracking extent in this

case but it is to be recognized that carbon pick up is possible.

EVALUATION OF THE UNMIXED ZONE IN CN-3MN SMA BOP WELDS (INCONEL 112 FILLER METAL) AND COMPARISON OF

CORROSION BEHAVIOR TO AL-6XN WROUGHT MATERIAL

1. Metallography

The unmixed zone samples used for the metallographic evaluation were from

transverse cross-sections in the CN-3MN SMA (BOP) weldments (Inconel 11 2

electrodes). Glyceregia and oxalic acid etchants were chosen for revealing the

unmixed zone through the electrolytic etching approach. The typical appearance of

the unmixed zone in the CN-3MN SMA welds (Inconel 112 filler) is presented in

Figure 32. Glyceregia revealed the unmixed zone as a dark-etching region

compared to a light-etching casting and composite zone as shown in Figure 32a.

Oxalic acid revealed the unmixed zone as light-etching together with light-etching

casting as presented in Figure 32b. It is evident that both Glyceregia and Oxalic Acid

etchants are remarkably effective in revealing the unmixed zone.

SEM evaluations of the unmixed zone were also performed on the transverse

cross-sections of CN-3MN welds (Inconel 112 filler). An SEM micrograph of the weld

interface is presented in Figure 33. An automated EDS program was used for the

quantitative analysis of the unmixed zone phenomenon. The distribution of the

alloying elements across the unmixed zone was determined at 7.1 µm intervals.

Figure 34 shows the chemical composition profile for Ni, Cr, Mo, and Fe across the

unmixed zone along line AB (Figure 33). It is evident that the unmixed zone has the

same chemical composition as that of the base casting.

The distribution of the unmixed zone was examined along the weld interface

of the CN-3MN SMA BOP weldment (Inconel 112 filler). The measurement of the

width of the unmixed zone was performed on the transverse cross-sections. The

typical appearance of a transverse cross-section of the CN-3MN welds is shown in

Figure 35. The width of the unmixed zone was measured perpendicular to the weld

interface from the weld face to the root. The variation in the width of the unmixed

zone along the weld interface is presented in Figure 36. The widest unmixed zone is

located near the weld root. The unmixed zone distribution behavior in the CN-3MN

with Inconel 112 filler is the same as that of AL-6XN sheet welds with Inconel 112

filler. However, compared with the AL-6XN sheet material, the unmixed zone in the

CN-3MN weldment extends further toward the sample surface. It is to be noted that

the CN-3MN weld was made on a 2" thick block but the AL-6XN sheet welds were

deposited on 0.063" thick sheets with a copper backing.

Evaluation of the solidification behavior of the unmixed zone was conducted

on the CN-3MN BOP welds (Inconel 112 filler) using OLM. The specimens were

extracted in both transverse and face orientations. For the evaluation of the unmixed

zone solidification behavior, the specimens were polished to 0.05 µ m and

electrolytically etched with Glyceregia to reveal the solidification substructure in the

unmixed zone and the structure of the CN-3MN casting as shown in Figures 37 and

38. Figure 37 (a & b, bright & dark field images) shows the solidification behavior of

the unmixed zone in the transverse orientation, and Figure 38 (also bright & dark

field) in face view orientation. A coarse dendritic solidification substructure is evident

in the CN-3MN casting. Comparing the dendritic solidification orientation of the

casting, it is evident that solidification direction in the unmixed zone is in the same

orientation as in the base metal. This observation demonstrates that the unmixed

zone solidification is epitaxial.

2. Evaluation Of Corrosion Behavior Of The Unmixed Zone

2.1. Pitting Corrosion Behavior

Pitting initiation tests were performed on CN-3MN SMA BOP welds with

lnconel 112 filler. The "Yellow Death" solution (4% NaCl +1% Fe2(SO4)3 + 0.01 M

HCI) was chosen for the pit initiation study. The "Yellow Death" solution was

stabilized with sequestrant 0.01 M Na2EDTA (sodium ethylenediamine tetraacetate,

which is practically non-corrosive for both stainless steels and nickel base alloys) for

24 hours. No precipitates formed in the stabilized "Yellow Death" solution up to

85°C. The specimens, approximately 1 " square, extracted from the CN-3MN welds

in the transverse orientation, were polished to 0.05 µm and electrolytically etched

with oxalic acid to reveal the unmixed zone in the transverse orientation. The pitting

initiation test was started at 70°C and temperature was increased at 2°C increments.

Pits initiated in the core of the cellular dendrites in the unmixed zone at 76°C after 4

hours in stabilized "Yellow Death" solution as shown in Figure 39. No pitting initiated

in either the composite fusion zone or the base metal under this condition. It is

evident that the core of the cellular dendrites in the unmixed zone has the lowest

pitting corrosion resistance. It is anticipated that the reduction in corrosion resistance

is associated with Mo segregation in the unmixed zone. Compared to the pitting

corrosion behavior of cast CN-3MN, wrought AL-6XN presents similar corrosion

behavior with regard to pitting as shown in Figure 40. In addition, pitting initiation in

CN-3MN may be exacerbated by cracking since cracks that exist in base casting can

provide crevice conditions.

The Mo segregation behavior in the unmixed zone was quantitatively

analyzed using the CN-3MN welds. For the examination of Mo segregation in the

unmixed zone, an automated EDS program was used with a 4 µm interval. The Mo

content was measured along line XY as shown in Figure 41. The variation in the

content of Mo across several cellular dendrites in the unmixed zone is presented in

Figure 42, and the average Mo content of the base metal (6.3 wt %) is also plotted in

the same figure. The lowest Mo content is found in the core of cellular dendrites

(points B and D in Figures 41 and 42). On the other hand, the Mo content at the

cellular dendrite boundaries is higher than the average Mo content in the base metal

(points A and C in Figures 41 and 42). It is to be noted that Mo content in the core of

cellular dendrites (approximate 4.7 Wt%) is much lower than the average Mo content

in the base casting (6.3 Wt%). It is evident that Mo segregation occurred during in

situ resolidification of the unmixed zone and this plays an important role in the

reduction in the corrosion resistance of the unmixed zone.

2.2. Crevice Corrosion Behavior

Crevice corrosion tests were performed on CN-3MN GTA autogenous welds

using the UT crevice corrosion fixture as shown in Figure 43. The specimens

extracted from the autogenous welds were assembled in the UT crevice corrosion

device with a torque of 32 in-oz and immersed in 6% FeCl3 for 72 hours at each

desired temperature. Based on information that a CCCT of 43°C is characteristic for

AL-6XN base metal in the LaQue crevice corrosion test (72 hours, 6% FeCl3 solution)

(Allegheny Ludlum Corporation), the temperature was chosen as 45°C. After 72

hours at 45°C in 6% FeCl3, corrosion occurred in base casting, HAZ, and in the weld

fusion zone. Thus, the test temperature was reduced to 23°C (room temperature)

and then to 0°C during further crevice corrosion tests. The weight loss of the tested

specimens was documented as in Table 6.

Photographs were taken of the tested specimens after the 72 hours crevice

corrosion tests. The test results are presented in Figure 44 for 45°C, 23°C, and 0°C,

respectively. It is to be noted that for the 45°C test, crevice corrosion occurred over

almost the entire specimen length in terms of the cast base metal, HAZ (heat tinted

zone), and the weld fusion zone on the weld metal face side. The widest corroded

area is located in the weld fusion zone as shown in Figure 44. Thus, the autogenous

weld fusion zone has lower crevice corrosion resistance. For the samples tested at

23°C, crevice corrosion also occurred in base casting, HAZ, and the weld fusion

zone on the weld metal face side. But compared to the tested specimen at 45°C, the

23°C tested specimen corroded less seriously. For the crevice test at 0°C, corrosion

was observed in the "heat tint" region (heat tinted HAZ and weld fusion) on the weld

metal face side. This behavior implies that the oxide formed during welding has a

significant influence on crevice corrosion behavior. In addition, wrought AL-6XN has

a similar crevice corrosion behavior compared to CN-3MN as presented in Figure

45.

CONCLUSIONS

1.

2.

3.

4.

5.

6.

Intergranular type cracks present in the cast block are related to casting

solidification cracking. The extensive oxidation occurred during casting

solidification and solution heat treatment.

The Varestraint tests revealed that solidification cracking tendency in the

casting and weld is significantly related to the grain boundary segregation as

evidenced by extensive precipitation.

Weld deposition can extend and open pre-existing cracks (formed during

casting ).

The surface carbon gradient did not have any apparent influence on cracking

tendency.

Particles rich in Mg, AI, and Ca were found along grain boundaries as well as

in the CN-3MN matrix. Cu was also detected in association with particles on

grain boundaries. These particles can lower the grain boundary liquation

temperature. Silicates and Cr rich carbides found along grain boundaries can

also exacerbate cracking tendency.

Pitting initiated in the core of the cellular dendrites in the unmixed zone at

76°C (CPT) after 4 hours in stabilized "Yellow Death" solution. No pitting was

observed in weld metal (Inconel 112 filler) or CN-3MN base metal at same

temperature.

7.

8.

9.

10.

The lowest Mo content was found in the core of the cellular dendrites in the

unmixed zone. It is believed that the segregation of Mo occurs during re-

solidification in the unmixed zone and this plays an important role in the

reduction in the corrosion resistance of the unmixed zone.

At elevated temperature (45°C), crevice corrosion occurred over almost the

entire specimen length in terms of the base metal, HAZ (heat tinted zone), and

the weld fusion zone on the weld metal face side. The widest corroded area is

located at the autogenous weld fusion zone.

At 0°C, crevice corrosion was observed in the "heat tint" region (HAZ and

autogenous weld fusion zone). It is evident that the "heat tint" region is most

susceptible to attack.

Compared to wrought AL-6XN, CN-3MN behaves in an identical manner in

the crevice and pitting corrosion tests.