DEVELOPMENT OF SMAW ELECTRODE FOR WELDING OF DMR 249A & DMR 249B STEELS
Indian Navy and NMRL have taken up a project to develop suitable consumables through Indian consumable manufacturers Apart from developing consumables, understanding details pertaining to the welding procedural aspect like joint design, selection of suitable size of consumable, Interpass temperature, number of layers, heat input is essential as they can influence the properties of the weld metal. This paper details the developmental work under taken to develop electrodes for welding low alloy steel plates DMR 249A and very high tensile high strength DMR 249B grade steels and also establish the parameters to get desired properties.
The present steels are nickel bearing micro alloyed steel characterized by higher strength and superior toughness even at sub zero temperatures. These plates were hitherto being imported in quenched and tempered condition. Presence of alloying elements it yields higher strength with goodtoughness. The chemical and mechanical property of these steels is shown in Table-1 & 2.
Basically, the alloy additions after the transformation characteristics of the steel enable to achieving a higher strength and toughness after heat treatment. Fig.1 shows an isothermal diagram of typical Q&T steel. It can be observed from this diagram. The starting of the austenite to pearlite (or) ferrite transformation requires long durations and therefore even moderate / slow cooling produced martensite / lower bainite and avoid ferrite. (Ref-2)
Indian Navy in association with NMRL has specified the weld metal property requirements.The details of the various properties to be met by the weld metal are specified in the Table-3 & 4.With this understanding, the electrodes development work was under taken.
2 DEVELOPMENTAL WORKS
During development, several batches were produced, tested for all properties with boiler quality mild steel plates before standardizing. During formulating the suitable chemistry, following aspects were also kept in mind.
(a) Lower levels of impurity elements in the weld metal.
(b) Selection of suitable binder to get extra low hydrogen in the weld metal.
(c) Effect of dilution with base material.
(d) Sufficient percentage of Mn & Ni to get desired structure.
Thus electrodes meeting NMRL specification has been developed with the targeted chemistry.
3 EXPERIMENTAL STUDIES
For the purpose of this study, sufficient quantity of electrodes of size 3.15 and 4.0 mm was produced and used for internal laboratory tests and established the parameters with DMR plates. After qualifying the procedure at our R&D, it was sent for testing at NMRL, Ambernath. The procedure established with the consumable is detailed in Table-5 & Table-8. After satisfactory results were received from NMRL, few more batches were produced and established the repeatability at NMRL & user end CSL, Cochin also. The results obtained at user end are shown in Table-6, Table-7, Table-9 and Table-10.
The effects of various parameters are discussed in the following paragraphs.
4 RESULTS AND DISCUSSIONS
The results obtained through the above study were summarized as under:
From the results obtained we can conclude as follows:
(a) SMAW electrodes meeting NMRL specification requirements for welding these steelshave been developed.
(b) Lower IPT seems to have a beneficial effect especially when low temperature toughness properties are desired.
(c) Lower heat input produced better toughness.
(d) A judicious choice of various welding parameters produces desired results.
(e) Proper root face advised to qualify the welders.
(f) Diffusible hydrogen levels control is very important to get good ductility.