Weldability

Martensitic stainless steels can be welded in the annealed, hardened, and hardened-and-tempered conditions. Regardless of the prior condition of the steel, welding produces a hardened martensitic zone adjacent to the weld. In other words, the high-temperature HAZ will be in the "as-quenched" condition after welding, regardless of the prior condition of the material. In addition, the HAZ hardness is very much independent of the cooling rate over the temperature range experienced in common arc welding practices.Because such high hardness values render the material prone to cracking during fabrication, the selection of appropriate preheating levels and welding procedures is critical to the success of the welding process.

Hardenability of Ferrous AlloysThe capability of an alloy to be hardened by heat treatment is called its hardenability. The term hardenability should not be confused with hardness, which is the resistance of a material to indentation or scratching.Hardenability of ferrous alloys depends on the carbon content, the grain size of the austenite, the alloying elements present in the material, and the cooling rate.

The End-quench Hardenability Test- Lominy testA round test bar 100 mm long, made from the particular alloy, is austenitized-that is, heated to the proper temperature to form 100% austenite then quenched directly at one end with a stream of water at 24C. The cooling rate thus varies throughout the length of the bar, the rate being highest at the lower end, which is in direct contact with the water. The hardness along the length of the bar is then measured at various distances from the quenched end.

Hardness decreases away from the quenched end of the bar. The greater the depth to which the hardness penetrates, the greater the hardenability of the alloy. Each composition of an alloy has its particular hardenability band. Note that the hardness at the quenched end increases with increasing carbon content

AWS B4.0: Weldability TestingThe term weldability is the capacity of material to be welded under the imposed fabrication conditions into a specific, suitably designed structure and to perform satisfactorily in the intended service. The tests included:-the Controlled Thermal Severity (CTS) Test, -Cruciform Test, -Implant Test, Lehigh Restraint Test, -Varestraint Test, -Oblique YGroove Test, -Welding Institute of Canada (WIC) Test,-Trough Test, and -the Gapped Bead On Plate (GBOP) Test.

Their applications are summarized below:

Welding Dissimilar MetalsSelecting the appropriate welding process and thefiller metal requires careful consideration when joining dissimilar metals. The choice of both should bebased on metallurgical factors such as differences inthermal expansion coefficients between the weld metaland base metal, the effects of dilution on the weldmetal, and the possibility of changes in the structureof the materials after extended service at elevatedtemperatures.The shielded metal arc welding process has theadvantage in making dissimilar metal welds in that theamount of filler metal added is less influenced bywelder technique than the GTAW or GMAW processes.In GTAW, the welder can vary filler metaladdition to a very large degree.The gas tungsten arc welding process permits morecontrol over dilution than most other processes. Thegas metal arc welding (GMAW) process is sometimesused for joining dissimilar metals, but the proceduremust be carefully controlled to prevent excessive dilution.The submerged arc welding (SAW) process canalso be used, but again, procedures must be controlledto avoid excessive dilution from the joint sidewall.

Filler Metals.A variety of materials can be weldedusing nickel alloy filler metals. Stainless and carbonsteels, low-alloy steels, and high-nickel alloys areamong the possibilities.Either covered electrodes or bare filler metals areavailable and can be specified to suit equipment andskills.

Some of the most commonly used electrodes are listed in: ANSI/AWS A5.14, Specification for Nickel and Nickel Alloy Bare Welding Rods and Electrodes;and ANSI/AWS A5.11, Specification for Nickel and Nickel AlloyWelding Electrodes for Shielded Metal Arc Welding.

DILUTIONThe change in chemical composition of a welding filler metal caused by the admixture of the base metal or previous weld metal in the weld bead. It is measured by the percentage of base metal or previous weld metal in the weld bead.

The percentage dilution can be determined by measuring areas labeled A and B. Percentage dilution is then calculated as:

Dilution is usually considered as a percentage of the base metal which has entered into the weld metal. When two pieces of base metal are welded together, the final composition of the weld deposit consists of a mixture of base metal and filler metal. The portion of the base metal that has been melted in with the filler material and has diluted it may be expressed in percent dilution. This is determined by the following formula:

Typical values of dilution for various processes are:

Many factors affect dilution:The greatest dilution occurs when no filler metal is added. In this instance, all of the weld deposit is self-generated by the base metal. Similarly, a single-pass weld will have a higher percentage of dilution than a multi-pass weld. There is always considerable dilution in the root pass. The greater the amount of weaving, the greater the dilution.Dilution as low as 2% has been achieved with the plasma arc hot wire cladding operation utilizing two hot wires connected in series. The application was welding copper rotating bands on artillery shells.

What is the difference between the various Carbon Equivalent Formulae used in relation to hydrogen cracking?Carbon is the most important of all alloying additionsto steels because of the effects it produces on themicrostructure as the welds cool from the very hightemperatures associated with the deposition of weldmetal. This applies as much to the heat-affected zone(HAZ) of the plate as it does to the weld metal. Inaddition, when carbon equivalents are of concern, theyare generally related to the HAZ.Two of the most troublesome problems associatedwith fabricating steels are hydrogen-induced crackingand poor toughness or ductility. Both are aggravatedby a microstructure called martensite. Since martensiteis very hard, its presence can be inferred by measuringthe hardness of the HAZ, particularly in the coarsegrainedregions which are close to the weld deposit.Carbon has a profound and direct effect on hardness.Other alloying elements also affect hardness, althoughnot to the same degree. In total, they affect the facilitywith which a given hardness can be obtained in analloy steel. This is called hardenability.However, the most important use of this concepthas not been in predicting hardness, but predicting theminimumpreheat temperature needed to avoid the formationof the hard martensite. Since martensite is producedat higher cooling rates, anything that can bedone to reduce cooling rates can be beneficial towardavoiding that microstructure or a high hardness. Preheatisimportant because it has a very strong effect on the rate at which welds cool. Weldability, energy inputand cooling rates are important variables.

Carbon equivalent formulae were originally developed to give a numerical value for a steel composition which would give an indication of a carbon content which would contribute to an equivalent level of hardenability for that steel. These formulae were later extended to represent the contribution of the composition to the hydrogen cracking susceptibility of steel. They are also used as compositional characterising parameters for other properties that may be linked to hardness, such as toughness and strength.These kind of relationships originate from about 1940 when Dearden and O'Neill proposed a carbon equivalent formula to predict steel strength, hardenability and HAZ hardness[1]. In 1967, the International Institute for Welding (IIW) adopted a somewhat simplified form of Dearden and O'Neill's formula for hardenability which became a generally accepted measure of steel weldability - CEIIW

Since its adoption by IIW, the equation has been incorporated into a number of material standards and codes, including EN 1011-2:2001[2](replaces BS 5135-1984[3]) and in a modified form in AWS D1.1[4], with a "+Si/6" term added to the equation.Further development of Carbon equivalent formulae has taken place and several can be found in technical literature today. Three of the more common ones are Pcm, CEq and CEN. Ito and Bessyo[5]developed Pcmin Japan based on a wider range of steels than the IIW formula:

The CEq formula devised by Dren[6]has a similar appearance:

Both the Pcmand the CEq formulae were developed for low carbon steels for which the CEIIWis less suitable. Pcmis generally used for modern steels typically used for pipeline manufacture, where carbon contents are no more than ~0.11 wt%[7].However, it should be noted that the Pcmformula was derived largely from lower C low alloy steels.The CEN formula was proposed to evaluate the weldability of a wide variety of steels. For the higher C range, the values of CEN correlate well with carbon equivalents such as CEIIW, whereas for lower carbon steels the values are close to those of the CEq formula. CEN is given by:

Yurioka[8]illustrated a good correlation between Pcmand CEN for structural steels, low-alloy steels (Ni-Cr-Mo type) and carbon steels, provided the carbon content was less than 0.17 wt%. From this comparison the following relationship was derived:CEN = 2Pcm- 0.092 (C 0.17%)

For carbon steels, the values deviate from this relationship as the Pcmoverestimates the cold cracking susceptibility. Where the carbon content exceeds 0.17 wt%, there is a better correlation between CEN and CEIIW:CEN = CEIIW+ 0.012 (C 0.17%)Yurioka[8]grouped a number of carbon equivalents for the assessment of weldability, as follows:

GroupFormula

A

B

C

Group A are characterised by 1/6 as the coefficient of Manganese; Group B has Carbon as more important than the other alloying elements and is more applicable to modern steels; Group C includes interactions between Carbon and other elements.Other existing formulae that could be classified as Group B are the CEw developed by Cottrell[9], and the CET included in the standard EN 1011-2:2001[3]. Cottrell claimed that the CEw formula could improve the predictions of cracking by a factor of three, compared to CEIIW. The data on which the formula was based was collected from results published in open literature, and covered a wide range of composition and welding conditions. The resulting formula is as follows:

The CET formula is based on similar elements to the CEIIWformula with the exception of Vanadium, although carbon is considered to have more significance than the other elements: