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Welding-alloy-steelRisks, Challenges andSolutions with Effective, Practical Advice.What are Alloy Steels? Welding-alloy-steel needs first an answer to the above question. Alloy Steels are materials of special compositions, developed to permit the deployment of elevated mechanical properties that make them the most suitable selection for important applications like bridges, high rise towers and lifting equipment. By utilizing the most out of improved strength, hardness, ductility and impact resistance through innovative design, it is possible to build lighter structures with considerable economical gains. Welding-alloy-steel, Heat Treatable, Quenched and Tempered Alloy Steels is a challenging proposition that needs understanding and preparation. These steels have 0.25 to 0.5%C, that is medium carbon content, and typically up to 5% total alloy content. What are the dangers? The strength, hardness and ductility that can be developed are provided by hardening and tempering these steels to obtain the sought for martensitic structure. However whenever martensite deriving from Welding-alloy-steel heat cycles is still untempered, it is hard and brittle and prone to cold cracking under the effect of internal stresses. Therefore the same favorable qualities that make these materials useful for demanding applications render them more susceptible to cold cracking during Welding-alloy-steel. The most important parameters, heat input and cooling rate, affecting Welding-alloy-steel should be addressed whenever the carbon present and the "alloy content" (meaning by that the sum of the percentages of all alloying elements) have a major influence upon the behavior of the material under the thermal cycles associated with welding. Annealed or overtempered conditions are preferred for easier Welding-alloy-steel while full deployment of properties is obtained by performing heat treatment once all welding operations have been completed.
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Designations and basic metallurgy. Some of these steels are known by the accepted AISI-SAE designation, as 13XX, 40XX, 41XX, 43XX, 46XX, 51XX, 61XX, 86XX, where the last two digits XX indicate the carbon content, expressed in hundredths of one percent, can be anything between 18 and 50. Some basic steel metallurgy facts should be remembered when Welding-alloy-steel. The Carbon level establishes the hardness and brittleness that will be shown by the martensitic structure. This is produced by fast cooling after austenitizing (that is after heating the steel above the transformation temperature where ferrite is changed to austenite). The problem is further aggravated by the higher hardenability due to high alloy content of the steels, meaning their tendence to harden, by forming martensite, even at larger sizes and slower cooling rates that would not influence other less alloyed carbon steels. Higher hardenability is what differentiates alloy steels from carbon steels of the same carbon content and represents also the most important Welding-alloy-steel problem. This means, as seen above, that hard martensitic structures are reached even with slow cooling from welding temperatures. Weldability, understood as the ease of welding without cracking, decreases in steels as the hardenability increases. This means that the higher the carbon and the alloy content, the higher the risks of cracking, if suitable precautions are not implemented. A useful tool. The concept of Carbon Equivalent was developed in an effort to reduce to a single number the influence of the contribution of the various alloying elements on the difficulties encountered in Welding-alloy-steel, therefore making the problem more tractable. One of the accepted empiric formulas equates the carbon equivalent to
the sum of the percentage of each element divided by a certain factor
as follows:
Carbon Equivalent The usage of this formula is intended to provide a rule of thumb for deciding if and what special provisions should be implemented for Welding-alloy-steel: for CE equal to or less than 0.40, no provisions are required. For CE more than 0.40 but less than 0.60 some preheating should be provided before welding. For CE more than 0.60 both preheating and postheating should be applied. It is evident that this approach to weldability evaluation oversimplifies the issue and overlooks other factors, like additional elements not accounted for, thickness, restraint of the joint, nature of the filler material, thermal gradients developed, all of which contribute to and may even decide the outcome of a Welding-alloy-steel procedure. For any real application the complex of all the conditions involved should be evaluated. It is equally important to clean thoroughly all materials involved, base metal, consumables, fixtures and accessories, from grease, paint, moisture, rust, dirt and any other contaminant. The risks of hydrogen. For Welding-alloy-steel, hydrogen is the most dangerous of the gases because it can induce underbead cracks. It can usually be introduced by moist electrode covers or other conditions associated with poor weld preparation and poor housekeeping. It can be absorbed in the melt in atomic form at elevated temperature, and then be rejected when the solubility drops at lower temperature, with substantial pressure increase in the passage to molecular form. Although appealing for its simplicity, this theory has been recently questioned and displaced by another model involving the hypothesis of the presence of preexisting defect sites in the material. There, under stress, hydrogen preferably diffuses, reducing the local cohesive strength. Failure would occur when this strength falls below the intensified stress level. Hydrogen evolves in the newly formed cavity and the process is repeated. Because of the tendency of cold cracking, exhibited by alloy steels, it is of utmost importance to minimize the possibility of hydrogen embrittlement, by using only low hydrogen consumables. Low hydrogen electrode covers for limiting hydrogen pick-up are formulated for Welding-alloy-steel and highly constrained joints; they need to be stored and kept dry to minimize moisture absorbance. Applicable processes. All the common arc processes are applicable in Welding-alloy-steel, the selection being determined mostly by economic and practical considerations. However certain precautions must always be considered: low hydrogen consumables, preheat and postheat to drive hydrogen away and to avoid cold cracking, besides controlling the microstructures formed. For these reasons, Shielded Metal Arc Welding is performed with low hydrogen electrodes. The purpose of the selection of filler metal is to match in the weld metal not so much chemistry and composition, but rather the mechanical properties obtainable after proper heat treatment. Some electrodes not covered by accepted Standards are offered for special purposes by manufacturers. Gas Tungsten Arc Welding is considered best capable of controlling hydrogen content to the minimum and is therefore the process of choice for critical Welding-alloy-steel applications. See Tig Welding Tips (Opens a new page) for useful advice. Both gas shielded manual processes (GTAW and GMAW) provide good control of chemistry and cleanliness. When higher productivity is required then mechanized processes as above or FCAW and SAW can be implemented, usually with more consistent quality. Some experts however question the capability of manufacturers to control the moisture content in the flux, and therefore advise against FCAW in critical applications.
Filler metals. Filler metals should be purchased from reputable manufacturers who are familiar with welding requirements and take care not only of the composition but also of surface finish and cleanliness of their materials. Flux cored wires can be supplied with compositions adjusted to give in the weld properties similar to those of base material, after hardening and tempering. Manufacturers should be questioned to satisfy special requirements. When the behavior is more important then the chemistry of the base metal, it is customary to select lower carbon but higher alloy filler metal to provide the required properties while easing the cracking problem. Some of these electrodes provide as welded hardness close to that of fully treated base metal even with lower carbon content. When, in particular cases, the deployment of full quenched and tempered properties in the weld metal is not a necessity, the assembly can be put in service after stress relieve only. If appropriate, a non hardenable electrode may be selected, like an austenitic stainless or a nickel alloy: the lower strength and higher ductility contributes to obtain crack free welds. From this exposition it results that the selection of the proper filler metal electrode is governed by the design strength level of the welded joint. This requirement should be taken care of, while the other need to minimize cracking should suggest the selection of the consumable providing maximum ductility. An Article on selection of Filler metals for welding Alloy Steels was published in the December 2004 Issue of Practical Welding Letter #016. To read it click on PWL#016. An Article on Filler Metal for welding High Strength Steels was published (4) in Issue 50 of Practical Welding Letter for October 2007. Chemistry of the weld. In general one should be aware of the fact that the deposited weld material in Welding-alloy-steel may differ from the composition of filler metal, because of dilution with base metal and because of arc transfer efficiency, which depends on how well the elements are transferred across the arc. Therefore not all the elements in consumable electrodes are present in the weld bead in their original percentage, while filler wires used with nonconsumable electrodes and fed directly to the weld puddle, are more likely to pass unaltered in the weld. A considerable latitude of selection is often given to the welding specialist, who can choose the filler to provide for those characteristics that will give the best overall performance, even with a composition differing from that of the base metal. In particular better weldability is sometimes achieved by employing a filler composition which decreases the hardenability of the weld.
Covering the modifications of weld pool metal
as a function of the influence of Base Metal
on the composition of the filler metal
an Article on Pickup, Dilution and Recovery
was published in the November 2004 of
Practical Welding Letter Issue No. 15 in Section 2. To receive free of charge all the issues of Practical Welding Letter as they are published, please subscribe.
Coefficient of Thermal Expansion. Another factor to be taken into account is the coefficient of thermal expansion, especially for dissimilar joints, where a suitable filler metal should be selected to accomodate for different thermal behavior, and to absorb without cracking the internal stresses likely to develop in the joint because of this difference. The joint could be weakened by carbon depletion in the base metal caused by certain filler metals. Another filler having less tendency to deplete carbon should be considered if the joint mechanical properties, to be verified by tensile and bend tests across the weld, are important for the application. Other harmful elements. Elevated contents of sulfur or phosphorus, which are not included in the formula for Carbon Equivalent, may contribute to the appearance of hot tears in the weld. By hot tears one means cracks, caused by internal stresses, appearing at or near the end of the solidification process, while the material is still hot and weak. Sometimes the adverse effect of sulfur can be counteracted by providing a filler material with increased Manganese content, which contributes to produce harmless manganese sulfide inclusions, thus resolving the problem of sulfur generated hot shortness. Gases trapped in the weld are revealed by the presence of porosity which is enhanced when the solubility at cooler temperature is lower than that in the liquid metal or at elevated temperature. Controlling microstructure. Welding-alloy-steel provides intense local heat which affects the structures present near the joint and induces those structural changes that have to be anticipated by knowing the chemistry of the base metal, the shape and dimensions of the structural elements and the cooling rate. As already pointed out, hardness and brittleness go together. Therefore if the conditions (carbon and alloy content) are such that hard and brittle martensitic microstructures are to be expected upon cooling from Welding-alloy-steel temperatures, with the concurrent risk of development of cracks, then modification of the cooling rate is to be implemented, mostly by preheating, to prevent the hardest structures from forming, or to temper them to lower and harmless hardness levels whith increased ductility. Heat input is a major factor involved in the success of Welding-alloy-steel. While the exact knowledge of net heat input applied may not be available because of heat losses that are difficult to account for, a general appreciation of its effects may help in evaluating the possible outcomes of procedure changes. An Article on Heat Input was published in the June 2004 Issue No. 10 of Practical Welding Letter. Click on PWL#010 to read it. An Article on Heat Flow was published in the issue No.14 of Practical Welding Letter of October 2004. To see the Article click on PWL#014. Preheat. In any given situation of joints presenting certain thicknesses and configurations, heat cycles affecting martensite formation of base metal near the weld are influenced both by preheat temperature and by heat input. As a precaution all hardenable steels should be preheated to decrease the cooling rate after welding. In general the higher the preheat temperature and the lower the heat input, the conditions are more favorable for limiting martensite formation and its hardness, hopefully contributing to higher quality welds. A similar result can be achieved sometimes simply by multiple pass Welding-alloy-steel, where successive beads temper and retard cooling of previous ones, with the benefits indicated above. If however the heat input provided by Welding-alloy-steel is not sufficient for keeping the structure as hot as needed, then external heating means must be implemented to assure the interpass temperature required. Adequate preheating must be provided in any case for the first, the root pass of Welding-alloy-steel, which is also the most crucial. The importance of preheating increases with the thickness of the base metal because of the rapid self quench capability, and with the rigidity of the welded structure because of the derived constraints. For Welding-alloy-steel designated as structural steels and high strength plates where specifications prescribe minimum yield strength in as rolled or in normalized condition, preheating is almost always required, together with filler material of the low hydrogen type which must be kept dry or baked before use. Tables are available giving recommended preheat and interpass temperatures for Welding-alloy-steel, based on chemistry of base metal and thickness of the structure elements. One such Table (No. 15) can be found at page 672 in ASM Handbook Vol. 6, in the Chapter on Welding of Low Alloy Steels. ASM Handbook : Welding, Brazing & Soldering The temperatures covered by the above Table range from a minimum of 40 0C (100 0F) for low carbon (0.2%C) and thin sections (less than 13 mm = 1/2") to a maximum of 370 0C (700 0F) for medium carbon (0.5%C) and thick sections (over 50 mm = 2"). A short note on References on Preheating was published in Section 11.1 in the June 2004 Issue No. 10 of Practical Welding Letter. Click on PWL#010 to see the References. An Article on Preheating Alloy and Tool Steels was published in Section 2 of Issue 37 of Practical Welding Letter for September 2006. To read the article click on PWL#037.
Postheat. Also known as Post Weld Heat Treatment (PWHT), this procedure is used to influence the structure and the properties obtained in the weld and in the heat affected zone (HAZ). By implementing proper provisions after welding one can retard the cooling rate after Welding-alloy-steel. The purpose is to prevent the martensite transformation by keeping the temperature high until other less hard structures are formed, or to temper the martensite already formed if it could not be avoided. Putting immediately the welded structure in a furnace, or covering the weld with some insulating material, or applying a flame from a burner are some of the usual procedures. An Article on Post Weld Heat Treatment (PWHT) was published in the May 2004, Issue No. 09 of Practical Welding Letter. Click on PWL#009 to read it. An Article on t8/5, the cooling time from 800 0C to 5000C, was published (2) in Issue 51 of Practical Welding Letter for November 2007. Click on PWL#051 to read it. An Article on Welding a Shaft was published (3) in Issue 52 of Practical Welding Letter for December 2007. Click on PWL#052 to read it. An Article on the "R" Stamp was published (11) in Issue 54 of Practical Welding Letter for February 2008. An Article on Type IV Cracking was published (7) in Issue 55 of Practical Welding Letter for March 2008. To receive free of charge all the issues of Practical Welding Letter as they are published, please subscribe.
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