Chemical Composition Variations in Shielded Metal Arc Welds Metal transfer droplet size, which changes with heating temperature, is discovered to be a factor varying chemical composition

BY A. Q . BRACARENSE A N D S. LIU

ABSTRACT. The use of shielded metal arc (SMA) welding can result in chemical composition variations along the weld length. Manganese and silicon, c o m m o n l y found in low-carbon steel welds, change in composition with weld position. This research was performed to better characterize the composition variations observed in structural steel welds and to understand the controlling factors that determine the extent of these composition changes. Single bead-on-plate and multipass welds were performed and analyzed. Manganese, silicon, and oxygen contents showed significant variation along the weld length. Hardness measurements and microstructure confirmed the strong effect of the composition change. To determine the cause of such composition variations, additional experiments were carried out w i t h the w e l d ing arc established between the electrode and a water-cooled copper pipe. The individual metal droplets were collected in water and processed using standard particulate materials processing techniques to remove the slag covering. The droplet size distribution was determined and related to the composition variation and position along the weld length. The results indicated that electrode preheating caused a change in the size of the droplets transferred during welding. At the beginning of welding, the electrodes were not heated as much and small size droplets predominated. The fine droplets, with large surface areato-volume ratio, experienced more complete deoxidation reactions and large losses in manganese and silicon. As elecA. Q. BRACARENSE and S. LIU are with the Center for Welding and Joining Research, Colorado School of Mines, Golden, Colo.

trade preheating becomes more intense, globular transfer with large droplets replaced the small droplets. Chemical analysis showed that more manganese and silicon were transferred across the arc to the weld pool. Introduction The shielded metal arc (SMA) welding process is probably one of the most versatile methods for joining steels. It is inexpensive, simple, and requires minimum welding skills in most applications. An SMA electrode consists of a metal core rod and a "clay-like" covering of powdered minerals such as fluorides, carbonates, oxides, organic materials, and alloying additions. A silicate binder is used to help extrude the flux ingredients onto the metal core rod. Subsequent baking of the electrode removes the moisture from the covering and forms a hard covering over the metal rod. During welding, both base metal and electrode are melted by the heat gener-

KEY WORDS SMAW Chemical Comp. Varies Covered Electrodes Structural Steel Electrode Heating Chemical Analysis Metal Transfer Mode Weld Metal Manganese Weld Metal Silicon Arc Physics

ated from the arc. The transfer mode of liquid metal from the electrode tip to the weld pool in SMA welding is often difficult to establish without special experimental techniques because of the fume and slag present (Ref. 1). However, it has been shown that globular transfer occurs in SMA w e l d i n g . Large droplets of l i q uid metal, at the size of the electrode diameter or bigger, grow at the tip of the electrode, detach and fall to the molten weld pool. Explosive transfer, before or after short circuiting of the metal droplet with the weld pool, was also observed in SMA welding. A showery spray of small droplets of liquid metal and slag fly across the arc including many projected outside the weld zone (Refs. 1, 2). Many factors are responsible for the transfer mode in SMA welding. The major ones are current, voltage, electrode diameter, melting temperature of the core material, coating thickness, and temperature of the electrode (Ref. 3). Few studies (Refs. 3-6), however, provide insights on the effect of electrode temperature on metal transfer and weld deposit properties. During welding, an electric current (I) passes from the electrode holder to the electrode and through the electrode to the arc column. As a result of the electrical resistance of the electrode, heating of the electrode occurs. Joule heating, which is given by the product of the square of the current (I2) and the electrical resistance (R), causes the electrode to heat up. Additionally, part of the heat of the plasma, w h i c h is given by the product of the electric current (I) and the arc voltage (V 0 ), also raises the temperature of the electrode. However, this contribution is minimum, because part of the arc energy is used to melt the tip of the elec-

W E L D I N G RESEARCH SUPPLEMENT I 529-s

TR

=

HEAT FROM JOULE EFFECT

HEAT FLOW . THROUGH TIP FLUX

COATING-

I

POWER

I SOURCE I V0

Q, LIQUID

=

DROPLET. V0

Fig. I — Schematic illustration

HEAT FLOW TO FLUX THE

of the thermal conditions

t r a d e a n d the base m e t a l , a n d part is lost b y r a d i a t i o n a n d by e v a p o r a t i o n of m a terial f r o m the surface of the electrode (Ref. 6). Figure 1 is a s c h e m a t i c illustration of the t h e r m a l c o n d i t i o n s e x p e r i e n c e d b y an S M A w e l d i n g e l e c t r o d e . From the e s t a b l i s h m e n t of the arc, the t e m p e r a t u r e of the e l e c t r o d e is e x p e c t e d t o increase as s h o w n in Fig. 2 . It is clear that d u r i n g n o r m a l w e l d i n g t h e t e m p e r ature of an e l e c t r o d e at a p o i n t r e m o v e d f r o m the arc c a n vary s i g n i f i c a n t l y , f r o m r o o m temperature to over 1000°C (1 832°F) (Ref. 3). C o n s e q u e n t l y , the m e l t rate of e l e c t r o d e a n d t h e m e t a l transfer are e x p e c t e d to c h a n g e w i t h w e l d i n g t i m e and position along the w e l d length. D u r i n g w e l d i n g , the length of the e l e c t r o d e , £, also d i m i n i s h e s , w h i c h d e -

=: ARC

FUSION

=

HEAT FROM PLASMA

NEAR

BOUNDARY

VOLTAGE

in a covered electrode (Ref. 6).

creases t h e J o u l e e f f e c t . H o w e v e r , t h e increase in t e m p e r a t u r e of t h e c o r e r o d m a t e r i a l leads t o an i n c r e a s e o f t h e res i s t i v i t y , p (Ref. 7), w h i c h d e s p i t e t h e e l e c t r o d e length decrease, the Joule h e a t i n g c o n t i n u e s to be s i g n i f i c a n t . Figure 3 s h o w s the increase of resistivity of s o m e c o m m o n steels w i t h t e m p e r a t u r e . Based o n this fact, Fig. 4 s h o w s s c h e m a t i c a l l y t h e d i s t r i b u t i o n o f the t e m p e r a t u r e of the metal c o r e rod a l o n g its l e n g t h , as p r o p o s e d b y W a s z i n k , ef al. (Ref. 6). Next to the electrode holder, the t e m perature of the c o r e r o d increases r a p i d l y t o a steady t e m p e r a t u r e . A t a s h o r t d i s tance f r o m the arc, the t e m p e r a t u r e of t h e e l e c t r o d e t i p increases r a p i d l y t o t h e m e l t i n g t e m p e r a t u r e . W a s z i n k , et al. (Ref. 6), e s t i m a t e d that the r a p i d t e m p e r -

Table 1 — Welding Conditions Used for the Three Electrodes Core rod diameter = 3.2 mm (% in.) Conditions Current (amperes) Voltage (volts) Travel speed (mm/s) Heat input (kj/mm)

E6013 | iA 23 2.05 1.5

E7018

El 2018

134 25 2.05 1.5

130 27 2.5 1.4

Table 2 — W e l d i n g Conditions Used in the Experiment w i t h E7018 Electrodes to Verify the Composition Variation and Its Dependence of Welding Current Core rod diameter = 3.2 mm (% in.) Conditions Current (amperes) Voltage (volts) Travel speed (mm/s) Heat input (kj/mm)

Lower Current 100 25 1.69 1.5

Higher Current 150 25 2.54 1.5

Table 3 — C h e m i c a l C o m p o s i t i o n in w t - % of t h e A 3 6 Plate a n d E 7 0 1 8 Electrode C o r e Rod Element Carbon Silicon Manganese

530-s I DECEMBER 1993

A36 Steel 0.1282 0.2637 0.9688

E7018Core Rod 0.1136 0.0094 0.4957

ature increase occurred at about 1 mm (0.039 in.) from the molten electrode tip. At the melting front, the core rod is much hotter than the surrounding covering. As the electrode heats up by Joule effect during welding, the portion of the electrode tip that experiences the transient temperature increase, A9, w i l l also increase. The electrode covering plays an important role by keeping the generated heat from the Joule effect and the heat conducted from the plasma inside the core rod. Since the electrical resistance of the electrode covering is several orders of magnitude higher than the metal core rod, Joule heating in the electrode covering is negligible because it can be considered that no current flows through the covering material (Ref. 6). In summary, with the melting of the electrode, more heat is generated and the electrode becomes hotter. The effects of electrode temperature increases during welding are various. It has been suggested (Ref. 3) that when an electrode is heated to high temperatures, the specific melting rate may experience a five-fold increase, w h i c h is the ratio between the latent heat of the hot metal and its heat of fusion. An extension of this observation may be the effect of electrode temperature on metal transfer mode, w h i c h may strongly influence the weld chemical composition. Additionally, it was found (Ref. 5) that oxygen content decreased w h i l e manganese and silicon contents increased as a function of the droplet growth time, as shown in Fig. 5. These observations seem to indicate that larger droplets will exhibit higher manganese and silicon content. This phenomenon must be associated with the deoxidation of the liquid metal droplet, w h i c h is controlled by kinetic factors such as temperature and droplet surface area. Experimental Procedure Bead-on-plate welds on A36 steel plates were conducted using E6013, E701 8, and E1 201 8 electrodes to verify the composition variations along the weld length. The welds were made with a linear heat input approximately equal to 1.5 kj/mm. The welding conditions for each electrode are shown in Table 1. Four additional sets of welding experiments were performed using E7018 electrodes to investigate the influence of Joule heating of the electrode on weld metal chemical composition. In the first one, bead-on-plate welds were prepared to verify the composition variation in the weld metal along the weld length at two levels of current. Composition changes were also correlated with d i l u t i o n , hardness and m i -

O

115 AMP

1200

/ O 1000 or:

m o AMF

/ /

/

/

/

/

. y