Transmission Electron Microscopy of Welded Cb-Microalloyed Steel The effects of resistance spot welding and GMA welding on the columbium precipitates in a microalloyed steel are investigated

BY P. L. M A N G O N O N

ABSTRACT. Transmission electron microscopy (TEM) of weld microstructures was conducted in 0.100 in. (2.5 mm) thick 50 ksi (348 MPa) minimum yield strength Cb-bearing controlled rolled HSLA steel, as well as in 0.100 in. (2.5 mm) thick SAE 1005 steel. The processes used were resistance spot welding and G M A W at heat inputs from 4.6 to 6.4 Joules/in. (0.8 to 0.25 Joules/mm). In spot welding, the influence of holdtime after welding on the microstructures and reprecipitation of columbium carbonitride was investigated. In GMA welding, modifications in the columbium precipitates were observed with changes in weld heat input. These modifications are similar to those observed in submerged arc welded plates reported in the literature; they did not impair the notch toughness properties of the G M A W steel as measured by a specially designed tension-impact test. Introduction Small additions of columbium (Cb) impart beneficial effects to the structures and properties of micro-alloyed HSLA steels. The effects are attained by controlled processing and may include: 1. Austenite grain refinement during soaking. 2. Keeping the austenite in the unrecrystallized condition during hot-rolling to produce very fine ferrite grains during transformation. 3. Increasing the hardenability when in solution in austenite.

P. L MANGONON is with the Department of Mechanical Engineering, Florida Institute of Technology, Melbourne, Florida.

4. Strengthening the ferrite by precipitating as very fine columbium-carbonitride particles. It is generally agreed that the carbonitride particles forming in austenite contribute insignificantly to the final strength. To be effective strengthener, the size of particles needs to be on the order of 2-5 nm (7.9 to 19.7 X 10 _ y in.) when precipitating in ferrite. For strip products, this size can be achieved by controlling the coiling temperature. The optimum microstructures and carbonitride particles produced after controlled processing are obliterated during welding. Depending upon welding conditions the carbonitride particles may dissolve or may undergo high temperature modifications. In a study to determine the influence of columbium and titanium in submerged arc welded carbon-manganese plate steels (Refs. 1, 2), plate-like eutectic-type carbide precipitates were observed at primary grain or dendritic cell boundaries in the weld. These platelike carbides are apparently different from those carbides which dissolve during soaking at about 1150-1200°C (2102-2192°F).

appeared to have lower melting points than that of MnS and may have resulted in hot cracking and reduced ductility. In another study (Ref. 4), eutectic linear columbium carbonitrides and eutectic sulfonitrides formed between 13501500°C (2462-2732°F). The latter study simulated the welding cycle by annealing specimens between 1200°C (2192°F) and the melting point, followed by slow or rapid cooling. These precipitate variations prompted the IIW to limit the columbium content to less than 0.05 wt-% as a precautionary measure.

Rather than dissolve, the plate-like carbides continued to precipitate at the boundaries of primary grains from 1300°C (2372°F) to the solidus temperature. These high-temperature carbides were also observed in heavy forgings and heavy castings of a steel containing columbium (Ref. 3). The plate carbides were considered responsible for the intergranular "rock-candy" brittle fractures in heavy castings. Columbium oxysulfides were also observed to form eutectic intergranular films in the HAZ and very close to the fusion line (Ref. 2). These sulfides

Materials and Procedures

The columbium precipitates were identified by electron microscopy (Refs. 1-4) after carbon extractions from fractured surfaces. All were observed in replicas made from either heavy forgings and castings or welded plates thicker than 15 mm (0.59 in.). The present paper reports on the observance of similar precipitates by TEM of thin foils from the 2.5 mm (0.100 in.) GMA welded specimens. The substructures produced after spot and GMA welding are also described for both a columbium-bearing and a plain carbon steel.

The materials were taken from commercially produced coiled hot-strip 2.54 mm (0.100 in.) thick Hi-Form 50 (Inland Steel's 50 ksi or 345 MPa minimum yield strength steel and described as HF 50 steel in the balance of this paper); SAE 1005 steels were also used. The compositions and tensile properties of these steels are shown in Tables 1 and 2, respectively. The HF 50 steel was primarily developed for automotive application, and its processing, structures, and properties have been described elsewhere

WELDING RESEARCH SUPPLEMENT 113-s

Table 1 - -Chemical Compositions of Steels, Wt-%

c Mn P S Si Cu Cb Al N Fe

0.106 in. thick SAE 1005 steel

0.100 in. thick HF 50 steel

0.04 0.33 0.008 0.017 0.003 0.06

0.06 0.33 0.008 0.029 0.081

—

0.036 0.046 0.006 Bal.

Table 3—Resistance Spot Welding Conditions

Material

Welding current, A

Welding time, Hz

Hold time, Hz

HF 50 (D (0.100 in.) (2)

15,900 15,900

30 30

50

SAE 1005 (1) (0.100 in.) (2)

10,100 10,100

30 30

Electrode force, lb

Electrode diameter, in

1

1800 1800

0.375 0.375

50 1

1600 1600

0.375 0.375

-

0.005 0.005 Bal.

(Ref. 5). T h e spot- a n d G M A - w e l d e d properties of HF 50 w e r e also the subject o f a n o t h e r presentation (Ref. 6). T h e electron m i c r o s c o p y that will b e described here w a s carried o u t o n t h e materials used in that presentation. The resistance spot w e l d i n g c o n d i t i o n s are listed in Table 3 w h e r e the principal variable studied w a s hold time. For G M A w e l d i n g , t w o w e l d i n g w i r e s w e r e used; their c o m p o s i t i o n s are s h o w n in Table 4 . W e l d i n g p r o c e d u r e consisted o f spacing the square edges o f the lightgage steels a b o u t 1.125 m m (0.045 in.) apart (the d i a m e t e r o f t h e w e l d wires) in a flat position w i t h n o backing material a n d n o preheat. T h e w e l d joint w a s m a d e using a stringer b e a d t e c h n i q u e t o o b t a i n c o m p l e t e joint p e n e t r a t i o n in o n e pass. T h e w e l d i n g c o n d i t i o n s are s h o w n in Table 5.

Table 4—Chemical Compositions of 0.045 in. Diameter Welding Wires, Wt-%

C Mn P S Si Mo Fe

A 340 320

„

Table 2—Tensile Properties of Steels

41,000

56,900

54,200

69,600

37 23.5

31 20

Fig. 1 — Weld cross sections: A — resistance spot weld: B — CMA weld. Base metal plate thicknesses are 0.100 in.

and — finally — e l e c t r o p o l i s h e d t o o b t a i n the foils. D u r i n g electropolishing, slight etching o c c u r r e d t o delineate t h e w e l d and h e a t - a f f e c t e d zones w h e r e t h e foils w e r e o b t a i n e d . T h e foils w e r e e x a m i n e d w i t h the Philips EM 300 m i c r o s c o p e o p e r ated at 100 kV.

FORMABIE SO

300

s

Yield strength (0.2% offset), psi Tensile strength, psi Total elongation, % Uniform elongation, %

0.07 1.77 0.015 0.012 0.41 0.42 Bal.

-

« 280

steel

0.10 1.19 0.011 0.024 0.53 Bal.

M

0.100 in. thick HF 50 steel

WireB

rolled surfaces; the w e l d e d joints, s h o w n in Fig. 1, are situated in the m i d d l e o f the surfaces. These w e r e g r o u n d t o r e m o v e equal a m o u n t s f r o m b o t h sides a n d p o l ished w i t h o n e - m i c r o n grit d i a m o n d t o a b o u t 0.5 m m (0.020 in.), t h e n chemically polished t o a b o u t 0.05 m m (0.002 in.),

Thin foils f o r transmission e l e c t r o n m i c r o s c o p y (TEM) w e r e p r e p a r e d f r o m specimens containing t h e original h o t -

0.106 in. thick SAE 1005

Wire A

|

260

2 Z £

240