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TitleDirect-Chill Casting Of Light Alloys: Science And Technology
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Page 1

DIRECT-CHILL CASTING
OF LIGHT ALLOYS

Page 211

HOT TEARING 199

the pattern is simple – a region of tensile stresses is located in the centre of
the billet, while the periphery experiences compression. Accordingly, hot
cracks usually appear in the central section of the billet.

The empirical criterion of hot-crack free casting can be formulated from
the condition of zero-tensile stress in the billet centre. The corresponding
casting speed was reported to be [146]

Vc D m h= 0 8. / ( / ), (5.18)

where D is the billet diameter in m.
Stress distribution in the ingot is more complicated with the zones of tension

concentrated closer to the short-side surface (along a′–c′ direction in Figure
5.22b) and corners. There is still a possibility of tensile stresses in the central
section of the ingot, but the magnitude of these stresses is usually considerably
lower than those at the short-side periphery (area c–c′–a–a′ in Figure 5.22b).
As a result of this stress distribution, hot cracks appear closer to the short side
of the ingot. The situation is further complicated by the deformation of the
wider (rolling) face of the ingot (along l direction in Figure 5.22b). As the
compressive stress is higher at the surface than in the interior, and is greater
in the middle of the rolling face than closer to the edges (where it in fact
changes to the tensile stress), the whole wider face of the ingot bends inwards,
exhibiting so-called pull-in distortion (Figure 5.23). Consequently, the part of
the solid shell closer to the mush experiences tension and is prone to cracking.
Hot tearing can appear if this tension region expands to the mush as shown
in Figure 5.24.

According to Livanov, susceptibility to hot cracking depends on the thick-
ness of an ingot and the casting speed, and does not depend on the multiplicity
(ratio of the width to the thickness of an ingot). It has been firmly established
that during casting in a low-head mould, a decreased casting speed eliminates
hot cracking in squared, rectangular, and round billets [145].

The casting speed below which hot cracks do not occur is determined by
the following formula [145]:





� = � � (5.19)

where Vc is the casting speed in mm/min, b is the thickness of the billet or
ingot in mm, m is the exponent (larger than unity, close to 1.5), and K is a
constant depending on the alloy.

In practice, modern ingot moulds are designed in a convex shape so as
to compensate for the pull-in of the rolling face and therefore reduce scalping
of the ingot before rolling. More on the mould design can be found in
Chapter 6.

Obviously, the increase in the casting speed that results in the deepening of
the sump and thinning of the shell will make the pull-in more pronounced with
the consequence of more hot tearing in the subsurface layer.

Page 212

200 SOLIDIFICATION PHENOMENA AND CASTING DEFECTS

In addition to “pull-in”, two other shape distortions are typical of DC
casting, namely, butt curl (lifting of the billet shell from the starting block) and
butt swell (protrusion of the billet shell) (see Figure 5.24). These defects occur
in the initial stage of casting close to the bottom end of the casting, which is
nick-named “butt”. Rapid cooling of the bottom end of the casting immedi-
ately after the exit from the mould – both from the starting block and from
the direct water jet – produces excessive thermal stresses that lift up the
corners (edges) of the ingot (billet) forming the butt curl. The occurrence of
butt curl can reduce the stability of the ingot standing on the starting block
and is therefore a potential safety hazard. The water may enter the formed
gap and vaporise forcing the ingot to “bump” and jump on the starting block.
Besides that, the partial loss of contact between the ingot and bottom block
will initially reduce the heat transfer with the possible danger of remelting. In
the worst case, butt curl can cause bleed-out, cracks, and hot tears. The butt
swell is a result of lesser apparent linear thermal contraction of the bottom
part of the ingot (billet). This part is formed in the mould closed by the starting
block when the sump is shallow. As soon as the casting exits the mould and

Figure 5.24. Shape distortions typical of direct-chill cast ingots. A view from the short
side of the ingot.

Page 422

410 INDEX

effective, 174, 187
vulnerable, 174, 180, 186–187, 190,

202, 224
rate, 10, 109, 144, 146–147, 150–154,

156, 163, 372
time, 5, 19, 147, 152–153, 338, 353
shrinkage, 161–162, 165, 167, 169, 173,

175, 187, 189–191, 193, 195, 236,
243, 346, 352, 354, 368

Solute accumulation, 106–110, 153
Specifi c heat, 25, 239, 263, 272, 274
Spinel, 31, 34, 59
Starting block, see Starting head(s)
Starting head(s), 225, 236, 251, 256, 269,

276, 282, 284–289, 297–298, 300–
302, 306, 311–312, 391, 395

Start up (phase of casting), 14–15, 192,
201, 351, 357

Steady-state, 9, 198, 201, 246–247, 271,
276, 306, 334, 340, 343, 351, 357,
364–365, 368

Stirring, 15, 26, 34–37, 40–42, , 66, 73, 82,
112, 120, 135–136, 158, 161, 169,
170, 219, 224–225, 255, 293

Strain
compressive, 165
critical, 185, 190
plane, 207, 215, 360
principal, 187
rate, 177, 181, 185, 188–189, 190–191,

193, 207, 215, 224, 353–354, 363
tensile, 203, 224
thermal, 175, 178, 182, 187, 189, 190,

203, 207, 224, 352, 361, 363
volumetric, 165

Stress
accumulated, 197
compressive, 158, 208, 212–213, 221,

223
concentration (concentrator), 194–196,

214
critical, 185
mean, 216
normal, 212–213, 351–352, 363
principal, 213–214, 216–218, 362–363,

366
relief (relaxation), 181, 190, 206–209,

218–219, 323–324

residual, 176, 197, 209, 210, 219, 224,
327, 342–343, 351, 360, 370

shear, 3, 16, 112, 123–124, 129, 213,
301, 363–365

tensile, 174, 194–199, 203, 206, 208,
212–215, 223–224, 351

thermal 10, 26, 173, 181, 189, 200, 205–
207, 211, 214, 219, 224, 247, 332,
340, 351–352, 364, 367–368

Strontium (Sr), 33–36, 64, 185
Structure

crystal, 105, 114–115, 119, 121, 124,
157

duplex, 152, 163, 346
fi r-tree, 65, 155–157
grain refi ned, 164

Submould cooling, see Water, cooling
Substrate, 22, 105, 108–110 112, 115, 117,

119, 123, 138, 147, 152
Sulphur (S), 50–51, 61, 63
Sump, 5–6, 10, 126, 128, 150, 153–155,

158, 161–162, 164, 166–169, 172,
175, 186, 191, 197, 199–202, 204,
213, 217–219, 224, 744

Surface cracking, 24–25, 197, 222–223,
226, 281, 285, 367

Surface segregation, 165, 222, 255, 258–
260, 272, 327, 368

Surface tension, 25–26, 78, 173, 182, 193,
223, 226, 264–265, 270, 356

Synthetic oils, 280

TAC crucible treatment system, 66,
83

Tap hole, 37–38
T-bar, 2, 16, 31, 237, 250, 281–282, 284,

289, 300, 322, 328, 395–396
Temperature gradient, 106, 146, 157–158,

224
Tensile strength, 177, 179, 182, 186, 203,

213–214
Thermal conductivity, 25, 40, 64, 89, 186,

242, 245, 252, 263, 279, 288,
305,335, 341, 370, 372, 374

Thermal contraction, see Contraction
Thermal expansion coeffi cient, 65, 122,

174, 186, 285
Thermal strain, 175, 182, 187, 189, 190,

203, 207, 224, 353, 361, 363

Solidifi cation (cont’d)

Page 423

INDEX 411

Thermal stress, 10, 26, 173, 181, 189, 200,
205–207, 211, 214, 219, 224, 247,
332, 340, 351–352, 364, 367–368

Thermite reaction, 34, 52, 312
Thermocouple(s), 32, 147, 153, 242, 332–

334, 338, 342, 369–370
Tin (Sn), 59, 61, 130
Titanium (Ti), 21, 33, 35–36, 57–60,

63–64, 67, 103–104, 109–120, 126–
138, 157–160, 169–171, 183, 185,
265

Trace element, 40, 57
Transient (stage of casting), 201, 306
Transition region, 10, 11, 150–151, 159,

161, 163, 166–167, 202, 224, 238,
346, 348

Trough, see Launder
Twin-roll caster/casting, 3–4, 17, 19–21,

24, 165
Hunter, 19–20
Jumbo 3C, 20

Twin-toll casting, 19

Ultrasonic processing, 128, 135
Undercooling, 105, 106–111, 124, 128,

136, 146, 149, 154, 338
constitutional, 106–110, 146
nucleation, 111, 134, 136
parameter, 107–108

Upsteam conduction distance
(UCD), 259–261, 263–265, 274,
373–374

Used beverage cans (UBCs), 39
Utilisation, 136, 387

Vanadium (V), 33, 35, 57–59, 64–65,
131

Vertical direct chill (VDC), 2, 6, 12,
15–16, 24, 32–33, 93, 191, 211, 221,
235–236, 243–244, 273, 275–276,
278–279, 281, 321–322, 327, 335,
389–391, 395–396

Waffl e, 34–36, 136
Wagstaff, 14, 255–256, 269, 272–273,

278
Water

cooling, 6, 14, 17, 22–23, 93, 218, 225,
237, 244–249, 252, 254, 264, 269,
280, 282, 303–304, 311, 334, 337 ;
see also Boiling mechanism(s)

fl ow rate, 149–150, 152, 155, 167, 198,
495

hardness, 557
impact velocity, 495
impingement zone, 158, 197
quality, 556
system, 439
temperature, 495
tower, 556

Water model, 49, 69, 291, 333, 368
Wetting, 105, 112, 122, 182, 248

Zero-ductility temperature, 177, 195
Zero-strength temperature, 177–178
Zinc (Zn), 1, 33–37, 36, 59–61, 63, 109,

114, 130, 137–138, 157, 169–170,
172, 176, 179, 180, 182–185, 223,
281, 369

Zirconium (Zr), 21, 33, 35, 37, 59, 64, 67,
104, 109, 112–115, 117, 120, 124–
130, 135–137, 157–158, 169–170,
172, 179–180, 185, 210, 324

Zunkel, 7

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