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There are a number of metallurgical factors that determine the weld-ability of a carbon or alloy steel. These include hardenweld-ability, carbon con-tent (and to a lesser excon-tent alloying concon-tent), weld metal microstructure, heat-affected zone (HAZ) microstructure, and preweld and postweld heat treatments.

Hardenability in steels is generally used to indicate austenite stability with alloy additions. However, it has also been used as an indicator of weldability and as a guide for selecting a material and welding process to

Table 12 Maximum safe forging temperatures for carbon and alloy steels of various carbon contents

Maximum safe forging temperature

Carbon

Carbon steels Alloy steels content, % °C °F °C °F 0.10 1290 2350 1260 2300 0.20 1275 2325 1245 2275 0.30 1260 2300 1230 2250 0.40 1245 2275 1230 2250 0.50 1230 2250 1230 2250 0.60 1205 2200 1205 2200 0.70 1190 2175 1175 2150

0.90 1150 2100 … …

1.10 1110 2025 … …

Fig. 34 Effects of compositions of three different steels on loads and pressures required for upset reductions of increasing severity at various temperatures

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avoid excessive hardness and cracking in the HAZ. Steels with high hard-ness often contain a high volume fraction of martensite, which is extremely susceptible to cracking during processing. Hardenability is also used to indicate the susceptibility of a steel to hydrogen-induced cracking.

Effects of Carbon Content and Alloying Elements. Traditionally, empirical equations have been developed experimentally to express weld-ability. Carbon equivalent (CE) is one such expression; it was developed to estimate the cracking susceptibility of a steel during welding and to determine whether the steel needs preweld and postweld heat treatment to avoid cracking. Carbon equivalent equations do include the hardenability effect of the alloying elements by expressing the chemical composition of the steel as a sum of weighted alloy contents. To date, several CE expres-sions with different coefficients for the alloying elements have been reported. The International Institute of Welding (IIW) carbon equivalent equation is:

(Eq 1)

where the concentration of the alloying elements is given in weight per-cent. It can be seen in Eq 1 that carbon is the element that most affects weldability. Together with other chemical elements, carbon may affect the solidification temperature range, hot tear susceptibility, hardenability, and cold-cracking behavior of a steel weldment. Figure 35 summarizes the CE CE C Mn

6  Ni 15 Cu

15  Cr 5  Mo

5 V 5

Fig. 35 Weldability of several families of steels as a function of carbon equipment. 1, Mo; 2, Cr + Ni + Mo + Si, and so on; 3, Cr or V or Ni + Si, and so on

Carbon and Alloy Steels / 183

and weldability description of some steel families. Because of the simpli-fication and generalization involved in Fig. 35, it should be used cau-tiously for actual welding situations.

Weldability of Low-Carbon Steels. At low carbon levels (less than 0.15% C), the steels are nonhardening and weldability is excellent. The bulk of the steels in this carbon range are used for flat-rolled products (sheet and strip), which may contain up to 0.5% Mn. Most of these steels are now aluminum-killed, continuous-cast product supplied in the cold-rolled and annealed condition. The lower available oxygen in the killed sheet makes it easier to arc weld without porosity formation.

Weldability of Mild Steels. In the range of 0.15 to 0.30% C, carbon, or mild steels, are generally easily welded, but because hardening is a possibility, precautions such as preheating may be required higher man-ganese levels, in thicker sections, or at high levels of joint restraint. The welding of sections that are more than 25 mm (1 in.) thick, particularly if the carbon content of the base metal exceeds 0.22 wt%, may require that the steel be preheated to approximately 40 °C (100 °F) and stress relieved at approximately 525 to 675 °C (1000 to 1250 °F).

Weldability of Medium-Carbon Steels. Steels containing 0.30 to 0.60% C can be successfully welded by all of the arc welding processes, provided suitable precautions are taken. The higher carbon content of these steels, along with manganese from 0.6 to 1.65%, makes these steels more hardenable. For this reason, they are commonly used in the quenched and tempered condition. Because of the greater likelihood of martensite formation during welding, and the higher hardness of the martensite formed, preheating and postheating treatments are necessary.

Low-hydrogen consumables and procedures should also be used to reduce the likelihood of hydrogen-induced cracking. The higher strength level of these steels may require the use of an alloyed electrode to match the base-metal properties. It may also be necessary to postweld heat treat the part in order to restore the strength and/or toughness of the HAZ.

Weldability of High-Carbon Steels. Steels containing 0.60 to 2.00% C have poor weldability because of the likelihood of formation of a hard, brittle martensite upon weld cooling. Steels of this type are used for springs, cutting tools, and abrasion-resistant applications. Low-hydrogen consumables and procedures, preheating, interpass control, and stress relieving are essential if cracking is to be avoided. Austenitic stainless steel electrodes are sometimes used to weld high-carbon steels. These electrodes will reduce the risk of hydrogen-induced cracking but may not match the strength of the high-carbon steel base metal.

Preheat and postheat will not actually retard the formation of brittle high-carbon martensite in the weld. However, preheating can minimize shrinkage stresses, and postheating can temper the martensite that forms.

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Successful welding of high-carbon steel requires the development of a specific welding procedure for each application. The composition, thick-ness, and configuration of the component parts must be considered in process and consumable selections.

Weldability of Free-Machining Steels. The 11xx and 12xx series steels contain large amounts of sulfur, phosphorus, or lead for improved machinability. Both series are difficult to weld because of solidification cracking and porosity formation.

Weldability of HSLA Steels. For all practical purposes, welding these steels is the same as welding plain carbon steels that have similar carbon equivalents. Preheating may sometimes be required, but postheating is almost never required.

Weldability of Quenched and Tempered Steels. These steels are fur-nished in the heat-treated condition with yield strengths ranging from approximately 350 to 1000 MPa (50 to 150 ksi), depending on the com-position. The base metal is kept at less than 0.22% C for good weldability.

Preheating must be used with caution when welding quenched and tem-pered steels because it reduces the cooling rate of the weld HAZ. If the cooling rate is too slow, the reaustenitized zone adjacent to the weld metal can transform either to ferrite with regions of high-carbon martensite, or to coarse bainite, of lower strength and toughness. A moderate preheat, however, can ensure against cracking, especially when the joint to be welded is thick and highly restrained. A postweld stress-relief heat treat-ment is generally not required to prevent brittle fracture in weld joints in most quenched and tempered steels.

Weldability of Heat-Treatable Low-Alloy Steels. Examples of heat-treatable low-alloy steels include AISI 4140, AISI 4340, AISI 5140, AISI 8640, and 300M. The high hardness of these steels requires that welding be conducted on materials in an annealed or overtempered condition, fol-lowed by heat treatment to counter martensite formation and cold crack-ing. However, high preheating is often used with a low-hydrogen process on these steels in a quenched and tempered condition, as in motor shaft applications. Preheating, or interpass heating, for both the weld metal and the HAZ are recommended. Hydrogen control is also essential to prevent weld cracking. Extremely clean vacuum-melted steels are preferred for welding.

Low sulfur and phosphorus are required to reduce hot cracking.

Segregation, which occurs because of the extended temperature range at which solidification takes place, reduces high-temperature strength and ductility. Fillers of lower carbon and alloy content are highly recom-mended. Preheat and interpass temperatures of 315 °C (600 °F) or higher are very harsh environments for welders because of the physical

discom-Carbon and Alloy Steels / 185

fort. However, the cooling rate must be controlled to allow the formation of a bainitic microstructure instead of the hard martensite. The bainitic microstructure can be heat treated afterward to restore the original mechanical properties of the structure. Specifications and procedures should be followed rigorously for difficult-to-weld materials.

Weldability of Chromium-Molybdenum Steels. The weldability of chromium-molybdenum steels is very similar to that of quenched and tempered and hardenable low-alloy steels. The major problem in the HAZ is cracking in the hardened coarse-grained region, as well as HAZ softening between Ac₁ and Ac₃. Reheat cracking during postweld heat treatment and long-term exposure in elevated-temperature service condi-tions also can cause severe problems. The appropriate preheat and inter-pass temperature should be selected, and low-hydrogen practice should be used.

Although a postweld heat treatment is not required for chromium-molybdenum steels with lower chromium contents and thinner gages, it is often conducted immediately after welding as part of the welding proce-dure. The postweld heat treatment of a chromium-molybdenum weldment is also referred to as a stress-relief heat treatment.