Heat impacted zone

Figure 4.13 a Precipitation of M3C and M7C3 carbides in grain boundaries of the welding heat-impacted zone, and b uneven distribution of fine MC caibides in grains boundaries of the welding... 

AU other carbon steel, low-intermediate, and high-aUoy steels, ferritic steels Base metal Deposited weld metal and heat-affected zone (See Note 1) 2. Except when conchtions conform to Note 2, the material shall be heat-treated to control its microstructure by a method appropriate to the material as outlined in the specification applicable to the product form and then impact-tested. (See Note 1.) Deposited weld metal and heat-affected zone shall be impact-tested. 

Select low-temperature steels for fracture-critical structural members designed for tensile stress levels greater than a ksl (40 MPa) and specify a minimum Charpy V notch Impact energy absorption of 20 ft-lb (27 J) for base metal, heat-affected zones (HAZs), and welds when the structures are exposed to low-ambient temperatures. Fracture-critical members are those tension members whose failure would have a significant economic impact. 

Rorvik (Ref 31) demonstrated that a zirconia-coated steel anvil retained more heat, with the workpiece allowing the tool to travel three times faster to obtain the same heat-affected zone width produced with a steel anvil. A statistical analysis of nine input parameters determined that cooling the anvil had a minimal impact on the friction stir weld in fact, tool rotation rate, travel speed, and tool depth were more important (Ref 24). Weld quality and performance is affected by differences in heat transfers observed when comparing the friction stir welding of flat plate versus extrusions (Ref 97). Extrusions typically have complicated cross sections, with features that quickly draw heat away from the friction stir weld. This dissipation of heat through the extrusion increases the tool heat input necessary to create a quality friction stir weld. 

Fig. 6.2 Charpy impact results. HAZ, heat-affected zone. Courtesy of Z. Feng, Oak Ridge National Laboratory...

Weld metal impact tests showed that all three filler metals used would meet a lateral expansion minimum of 0.381 mm at -196°C. The Inconel filler metals used in the nickel steel weldments easily met this requirement, while the stainless filler metal used was considered to be just adequate. All heat-affected-zone (HAZ) impact tests exceeded minimum values of 34 J absorbed energy and 0.381 mm lateral expansion at -196°C. 

The detailed nature of the specification was earlier mentioned. In fact the clarity of it concerning the steel itself left a great deal to be desired. Only four Izod impact tests were required, two with a notch in the weld and two with a notch in the heat affected zone. It was also stated that tests should be carried out at two temperatures, 32°F and 70 F. If this had been the case then only one test under each condition would have been made. In the event it was unfortunate that the lower temperature test was omitted, because it was the more critical one from the point of view of brittle fracture. If the reasons for these tests had been stated in the specification, then at least the CRB inspectors would have been able to approach their task with more understanding. The commissioners were of the opinion that the ambiguous use of the term BS 968 contributed to the series of misunderstandings that surrounded the supply of steel to the bridge. They were convinced that the train of events would have been quite different if the full specification of the steel required had been given without mention of BS 968. 

HAZ = heat-affected zone, COD = specimens for fracture toughness testing, CHARPY, CH = specimens for impact notch toughness testing, TENSILE = specimens for static tensile testing, G-1, G-2 = designation of surveillance specimen chains). 

CIT is a destructive test that utilizes test specimens machined from actual production test plates or sample materials. The test specimens can be machined for impact testing of all base metal, aU weld metal or the heat affected zone (HAZ). Weld samples can be taken from the root area or any other specific area of the weld metal. 

All four types of tests were made on the plate material in both the longitudinal and transverse directions. They were also made with the specimens transverse to, and centered on, the welds. In addition, notched tensile impact tests andCharpy keyhole impact tests were made with the notches located in the heat-affected zone next to the welds. Altogether, 14 series of tests were made on each alloy. Each test series covered the temperature range from -320 to 75 F. 

The severe conditions of the Charpy keyhole impact test did not produce a ductile—brittle transition in either the plate or heat-affected zone of alloy 5083, or in the weld deposit of alloy 5183. The properties of the plate and heat-affected zone were relatively insensitive to changes in temperature. There were, however, appreciable differences in the energy absorbed in comparable tests from each set. In order of decreasing energy absorption, the tests ranked as follows heat-affected zone, longitudinal plate, transverse plate, and weld deposit tests. 

In the case of weldments, Charpy V-notch impact testing of the base metal, the heat-affected zone, and the weld metal is also required to ensure that the weldment will function satisfactorily at the design temperature. Welding must be performed with care to ensure that the completed weldment is produced in accordance with standard procedures. 

High-Intensity Mixer. Mixers such as that shown in Fig. 18-44 combine a high shear zone with a fluidized vortex mixing action. Blades at the bottom of the vessel scoop the batch upward at peripheral speeds of about 40 m/s (130 ft/s). The high shear stress (to 20,000 s" ) and blade impact easily reduce agglomerates and aid intimate dispersion. Since the energy input is high [200 kW/m (about 8 hp/fE)h even powdery material is heated rapidly. 

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