|Title of Invention||
"A PROCESS OF PRODUCING SOUND WELDED JOINTS IN ATMOSPHERIC-AND MARINE-CORROSION RESISTANT STEEL BARS USING GMAW TECHNIQUE"
|Abstract||A process of producing sound v/elded joints in atmospheric- and marine-corrosion resistant steel bars using GMAW techinque, where the joints are adequately strong, free from welding defects and mechanically sound, characterised in the process comprises the following steps :- (i) cleaning the area of the steel bars to be joined adequately by known means; (ii) using the electrode wire of low hydrogen type conforming to AWS A 5.28 ER 80 S-G code; and (iii) optimising the conditions for butt-welding as: number of passes of the electrode wire -upto 8, shielding gas used - 100%C02, diameter of electrode wire - 1.2 mm; the bars joined -not preheated, the inter-pass are temperature -225°C (max) for all the passes, are voltage -22-28 volts, arc current - 150-200 A, and heat input from the arc - 8-15 KJ/cm, for Gr. A base metal, such as herein described, and 10-15 KJ/cm for Gr. M base metal, such as herein described, and for single lap-welding as : diameter of both Gr. A and Gr. M base metal bars - 16/20 mm, diameter of electrode wire - 1.2 mm, shielding gas used - 10096 C02, flow rate of gas - 7 lit./min, and preheating/post-heating/PWHT -not done. Reference: Figures 1 to 3 of the accompanying drawings.|
|Full Text||The present invention relates to a process of producing sound welded joints in atmospheric - and marine - corrosion resistant steel bars using GMAW technique.
The invention relates more particularly to a process for producing welded joints in 'thermo-mechanically treated1 (TMT) re-bars.
The examples of thermo-mechanically treated (TMT) atmospheric - and marine - corrosion resistant steel bars are respectively type Cu - TMT (500) and type Cu - Cr TMT (500), having chemical compositions as given in Table I. The joints produced by both butt - and lap - welding of such bars in the conventional "gas metal arc welding" (GMAW) method are sometimes not found to be adequately strong and free from defects, resulting in the failure of such bars in their end use applications.
The object of the present invention is to provide a process for producing adequately strong and defect-free welded joints in Cu-TMT (500) and Cu-Cr TMT (500) type steel re-bars, using gas metal arc welding (GMAW) technique.
In the conventional GMAW process of welding an arc is generated between a coated electrode wire and a base metal by momentarily touching the base metal by the electrode wire end to produce enough heat for melting both the base metal and electrode wire at the areas to be welded.
The molten electrode wire droplets are transferred through the arc into the pool of molten base metal and gets deposited on the base metal surface on solidification of both the molten electrode and base metals as the position of electrode
wire end is shifted along the "base metal surface to be welded. A protective gaseous shield is produced by an externally supplied gas which may be an inert gas or an active gas or a mixture thereof to envelop the arc. If the base metal is steel, the gas used for shielding the arc may be an active gas such as CO2 or a mixture of an inert gas such as argon and active gas
such as ^.
The main steps followed in the invented process are :-(i) joint preparation and selection of number of
Passes for butt weld; (ii) selection of electrode wire v/ithout having a
(iii) optimisation of the conditions for welding; (iv) evaluation of mechanical strength of and presence of
defects in the welded joints produced.
The invention is described fully and particularly in an unrestricted manner with reference to the accompanying drawings, in which
Figure 1 is a schematic lay-out of the invented GMA¥ method of welding;
Figure 2 is a schematic lay-out of butt-welding process; and
Figure 3 is a schematic lay-out of single lap-welding process.
Referring to Fig. 1, an arc A is produced between electrode wire E and the region B of the base metal M to melt both the base metal and electrode wire metal there. The droplets D of the molten electrode wire metal are deposited on the molten
base metal pool to form a pool of molten weld metal W there which on solidification produce a deposit of solidified weld metal S on the base metal M. A protective gaseous shield H is formed by the externally supplied gas such as CO2 or a mixture of CO2 and argon to shield the arc A, heated end of the electrode wire near the arc, molten droplets D and molten weld metal ¥, and region B and adjoining area of the base metal. The direction T of moving the electrode wire is shown to be from the left-hand end to the right-hand end of the base metal M.
The electrode wire used in the process is of a low hydrogen type conforming to AWS A 5.28 ER 80 S-G code.
Table - I
The adjacent ends X and Y of two bars M1 and M2 are butt-welded by adopting eight passes of the welding electrode
wire to produce the weld deposits marked 1, 2, 3, 4, 5, 6, 7 and 8 formed in sequence as shown in Fig. 2
The welding conditions followed for the two grades of base metal, namely Gr. A i.e. Cu-TMT (500) atmospheric -corrosion resistant and Gr. M i.e. Cu-Cr-TMT (500) marine -corrosion resistant are outlined in Table - II.
Table II Conditions for Butt-Weiding
Gas flow rate : 18 Ib/min; Shielding gas : 100?b C02
Generally neither any preheating of the base metals nor any post-welding heat treatment of the welded joints is found necessary.
Referring to Fig. 3, the adjacent surfaces of two bars M1 and M2 are joined by single lap-welding over the overlapping length V of the bars under conditions followed for the two types of bars, namely, Gr. A and Gr. M, as outlined in Table - III.
Table - III Conditions for Single lap-Welding
The mechanical strength of the butt- and single lap-welded joints produced in the process on the two types of steel bars, namely Gr. A i.e. Cu-TMT (500) and Gr. M i.e. Cu-Cr-TMT (500), has been tested and the typical test results are presented in Table - IV.
The presence of defects in the welded joints has been examined by LEE and MPI radiography. The joints welded in the process are found to be substantially defect-free.
Table - IV
Typical Mechanical Properties of the Butt- and Single Lap-welded Joints.
TS (MPa) Testing (NOT)
TS Minimum Position of LPI and
Type of Bars (MPa) Requirement Fracture MPI Radiography
#NSD : No Significant Defect
The results of Table - IV show that tensile strength of the joints produced in the process is appreciably higher than the minimum requirement stipulated for the satisfactory end-use performance of the bars containing the welded joints. The bend and charpy impact energy of the joints as shown in Table - V are also found to be satisfactory.
Table - V
Based on the results of the studies conducted, as
described hereinbefore, the optimum conditions of welding of
both Gr. A and Gr. M bars are as follows :
A. Butt Welding Optimum Conditions
(i) Electrode type : A¥S A 5.28 ER 80 S-G; (ii) Preheating of bars : Not necessary;
(iii) Shielding gas and flow rate : 100% C02 and 15-18 It/min;
(iv) Heat input : 10-15 KJ cm for electrode wire of
diameter 1.2 ran; (v) Current-Voltage - Heat Input Combination :
Inter pass temperature : 225°C max;
(vi) PWHT : Normally not required. However, in the
case of any specific requirement, the heat treatment
temperature is so chosen that the mechanical strength
of the welded joints is not affected unduly.
B. Single Lap Welding Optimum Conditions.
(i) Preheating of bars : Not required for both
Gr. A and Gr. M bars;
(ii) Heat Input (KJ/cm) : 8-15 for both Gr. A and
Gr. M bars;
(iii) Overlapped length of bars : 70 mm (minimum) for Gr. A
bars and 100 mm (minimum) for Gr. M bars.
1. A process of producing sound welded Joints in atmospheric- and marine-corrosion resistant steel bars using GMAW technique, where the Joints are adequately strong, free from welding defects and mechanically sound, characterized in the process comprises the following steps :-
(i) Cleaning the area of the steel bars to be Joined
Adequately by known means;
(ii) using the electrode wire of low hydrogen type conforming to AWS A 5.28 SR 80 S-G code; and (iii) optimizing the conditions for butt-welding as : number of passes of the electrode wire - upto 8, shielding gas used - 10096 C02, diameter of electrode wire - 1.2 mm; the bars Joined - not preheated, the inter-pass are temperature - 225°C (max) for all the passes, arc voltage - 22-28 volts, arc current -150-200 A, and heat input from the arc - 8-15 KJ/cm, for Gr. A base metal, such as herein described, and 10-15 KJ/cm for Gr. M base metal, such as herein described, and for single lap-welding as :
Diameter of both Gr. A and Gr. M base metal bars -16/20 mm, diameter of electrode wire - 1.2 mm, shielding gas used - 100% CO2 flow rate of gas -7 lit./min, and preheating/post-heating/PWHT -not done.
2. The process as claimed in claim 1, wherein the
chemical composition of the atmospheric-corrosion resistant
Gr. A base metal bars is (by weight %) :
C - 0.20 (max), Mn - 1.20 (max), S - 0.045 (max), P - 0.045 (max), Si - 0.40 (max), Cu/Cr/Ni - 0.25 (min) and Fe - balance.
3. The process as claimed in claim 1, wherein the chemical
composition of the marine-corrosion resistant Gr. M base metal
bars is (by weight %) : C - 0.20 (max), Mn - 1.20 (max), S - 0.045
(max), P - 0.045 (max), Si - 0.40 (max), Cu/Cr/Ni - 0.75 (min)
and Fe - balance.
4. The process as claimed in claim 1, 2 and 3, wherein
the overlapped length of Gr. A base metal bars is 70 mm (min)
and that of Gr. M base metal bars is 100 mm (min) in single lap-
|Indian Patent Application Number||2023/DEL/1997|
|PG Journal Number||04/2011|
|Date of Filing||21-Jul-1997|
|Name of Patentee||STEEL AUTHORITY OF INDIA LIMITED|
|Applicant Address||P.O. HINOO, DORANDA, RANCHI-834002, BIHAR.|
|PCT International Classification Number||B23K 31/02|
|PCT International Application Number||N/A|
|PCT International Filing date|