Page 36 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch01.qxd 9/29/05 4:59 PM Page 1.34

PROPERTIES OF STRUCTURAL STEELS AND EFFECTS OF STEELMAKING AND FABRICATION

1.34 CHAPTER ONE

1.26 EFFECTS OF WELDING

Failures in service rarely, if ever, occur in properly made welds of adequate design. If a fracture
occurs, it is initiated at a notchlike defect. Notches occur for various reasons. The toe of a weld may
form a natural notch. The weld may contain flaws that act as notches. A welding-arc strike in the
base metal may have an embrittling effect, especially if weld metal is not deposited. A crack started
at such notches will propagate along a path determined by local stresses and notch toughness of adja-
cent material.
Preheating before welding minimizes the risk of brittle failure. Its primary effect initially is to
reduce the temperature gradient between the weld and adjoining base metal. Thus, there is less like-
lihood of cracking during cooling and there is an opportunity for entrapped hydrogen, a possible
source of embrittlement, to escape. A consequent effect of preheating is improved ductility and notch
toughness of base and weld metals, and lower transition temperature of weld.
Rapid cooling of a weld can have an adverse effect. One reason that arc strikes that do not deposit
weld metal are dangerous is that the heated metal cools very fast. This causes severe embrittlement.
Such arc strikes should be completely removed. The material should be preheated, to prevent local
hardening, and weld metal should be deposited to fill the depression.
Welding processes that deposit weld metal low in hydrogen and have suitable moisture control
often can eliminate the need for preheat. Such processes include use of low-hydrogen electrodes and
inert-arc and submerged-arc welding.
Pronounced segregation in base metal may cause welds to crack under certain fabricating condi-
tions. These include use of high-heat-input electrodes and deposition of large beads at slow speeds,
as in automatic welding. Cracking due to segregation, however, is rare for the degree of segregation
normally occurring in hot-rolled carbon-steel plates.
Welds sometimes are peened to prevent cracking or distortion, although special welding sequences
and procedures may be more effective. Specifications often prohibit peening of the first and last weld
passes. Peening of the first pass may crack or punch through the weld. Peening of the last pass makes
inspection for cracks difficult. Peening considerably reduces toughness and impact properties of the
weld metal. The adverse effects, however, are eliminated by the covering weld layer (last pass).
(M. E. Shank, Control of Steel Construction to Avoid Brittle Failure, Welding Research Council,
New York; R. D. Stout and W. D. Doty, Weldability of Steels, Welding Research Council, New York.)

1.27 EFFECTS OF THERMAL CUTTING

Fabrication of steel structures usually requires cutting of components by thermal cutting processes
such as oxyfuel, air carbon arc, and plasma arc. Thermal cutting processes liberate a large quantity
of heat in the kerf, which heats the newly generated cut surfaces to very high temperatures. As the
cutting torch moves away, the surrounding metal cools the cut surfaces rapidly and causes the for-
mation of a heat-affected zone analogous to that of a weld. The depth of the heat-affected zone
depends on the carbon and alloy content of the steel, the thickness of the piece, the preheat temper-
ature, the cutting speed, and the postheat treatment. In addition to the microstructural changes that
occur in the heat-affected zone, the cut surface may exhibit a slightly higher carbon content than
material below the surface.
The detrimental properties of the thin layer can be improved significantly by using proper pre-
heat, or postheat, or decreasing cutting speed, or any combination thereof. The hardness of the ther-
mally cut surface is the most important variable influencing the quality of the surface as measured
by a bend test. Plate chemistry (carbon content), Charpy V-notch toughness, cutting speed, and plate
temperature are also important. Preheating the steel prior to cutting, and decreasing the cutting
speed, reduce the temperature gradients induced by the cutting operation, thereby serving to
(1) decrease the migration of carbon to the cut surface, (2) decrease the hardness of the cut surface,
(3) reduce distortion, (4) reduce or give more favorable distribution to the thermally induced stresses,
and (5) prevent the formation of quench or cooling cracks. The need for preheating increases with

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.