RU2376345C2 - Method of argon-arc treatment of welded joints for removal of welding temper - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: method includes heating by arc of section of pass from seam to main metal. It is implemented controlled-transfer heating by cycling for metal by base current I_b=20÷50 A and by pulse current I_p, value of which is selected from ratio:

,

where τ_b - duration of bas current, s, τ_p - duration of current in pulse, s, k=(1.3÷3.5) - coefficient, defining tolerance of parametre correlation of cycle, and with current pulse frequency f=0.1-0.3 Hz.

EFFECT: effective reduction of welding temper, increasing of mechanical properties of welded joint metal.

1 dwg, 1 ex, 1 tbl

Description

The invention relates to the field of electric arc processing of welds and heat affected zones to reduce residual welding stresses.

One of the undesirable effects of the thermal welding cycle is the presence in the welded joints of high values of residual welding stresses, which negatively affects the mechanical, corrosive properties of the welds and adjacent sections. In order to remove the high macro- and microstresses values requires the use of special measures, based on the heating of the heat affected zone above the allotropic transformation temperature A _C3 followed by slow cooling in the temperature range 920-350 ° C at a rate _OHL W = 1 ÷ 12 ° C / min .

Known methods for electric arc surface treatment of a metal product (RF patents No. 2144096 C23C 14/02, publ. 2000, RF patent No. 2165474 C23C 14/02, publ. 2001) carried out by an arc of reverse polarity, when the cathode spot is located on the surface of the workpiece. The physical feature of such processes is the chaotic movement of the cathode spot over the surface of the workpiece with high speed (Leskov GI Electric welding arc. - M .: Mashinostroenie, 1970, p. 101, 126), resulting in heating of the heat-treated sections does not exceed 150 -350 ° C, which does not allow to effectively reduce residual welding stresses. The use of various methods for controlling the movement of the cathode spot using magnetic fields provides only control of its direction of movement, not allowing the effective use of these methods for heat treatment of welded joints in the temperature range 920-350 ° C with an adjustable thermal cycle of heating and cooling, and determining their use, in mainly for cleaning the surface of metals due to the so-called “cathodic sputtering” effect.

There is also known a method of processing welded joints, in particular to prevent knife corrosion, which consists in local high-speed electric arc heating of the zone of connection of the weld with the base metal in a protective atmosphere while cooling the weld (USSR author's certificate No. 308077, C21d 9/50, publ. 1969). Heating is highly localized and is produced by an arc with a non-consumable tungsten electrode in an argon atmosphere. The electric arc moves at a speed of 30-35 m / h. Simultaneously with heating, the opposite side of the heated part of the seam is irrigated with water.

The thermal cycle of heating and subsequent cooling of the product at the required speed with this processing method is controlled by the heat removal rate by changing the flow rate of water for irrigation. Its disadvantages include, first of all, the limited possibilities of regulating the thermal cycle. Irrigation of the workpieces with water leads to intensive heat removal from the workpiece, which significantly increases the cooling rate. The regulation of the thermal cycle of the thermal effect on the metal by changing the flow rate of water allows changing the values of W _cool only upwards, which does not meet the task of reducing the residual welding stresses and, in the case of using carbon hardened steels, increases the level of residual stresses, deteriorates the mechanical properties and metal structure.

Closest to the invention relates to a method of argon-arc treatment of welded joints, including heating the arc section of the transition from the weld to the base metal. The edges of the seam and sections of the heat-affected zone are melted by a non-consumable electrode in argon (Asnis A.E., Ivashchenko G.A., Anderson Y.E. Experience in the use of argon-arc processing of welds in car building, "Automatic Welding", 1975, No. 11 p. 67-68). The absence of forced cooling reduces W _OHL, but the method can not effectively control the thermal cycle as high power arc and low speed its movement required for the necessary heat in the processing of thick material, cause large size of the weld pool and its volume, which adversely affects the value of the residual stresses. On the contrary, with a small thickness of the treated metal, even at low arc currents necessary to ensure the stability of the combustion and the processing process, overheating of the treated zone and a decrease in the achieved effect are observed.

The technical result of the present invention is to effectively reduce residual welding stresses, increase the mechanical properties of the weld metal.

This is achieved by the fact that in the method of argon-arc treatment of welded joints to relieve residual welding stresses, which includes heating the arc of the transition section from the weld to the base metal, pulse-arc heating is performed by cyclic exposure of the metal to a base current I _b = 20 ÷ 50 A and pulse current I _and , the value of which is selected from the ratio:

, А

, BUT

where τ _b - the duration of the base current,

τ _and - the duration of the current in the pulse,

k = (1.3 ÷ 3.5) - coefficient determining the tolerance of the ratio of cycle parameters,

and with a frequency of current pulses f = 0.1-03 Hz.

Heating and cooling of the metal during arc heat treatment is characterized by a thermal cycle at various points in the processing zone when the heat source passes (curve 1 in figure 1). The cooling rate of the metal after removal of the heat source is determined by the angle α _{1 of the} slope of the velocity vector W _{cool 1} at a point of interest to the time axis. When pulsed argon-arc heat treatment, the thermal cycle is characterized by pulsating cyclic heating and cooling (curve 2 in figure 1), and during the action of the current pulse the heating cycle is carried out, and during the flow of the base current the temperature of the heated zone decreases, and the arc during this period provides heating of the cooled metal, thereby reducing its cooling rate until the action of the subsequent current pulse. During the next pulse, re-heating of the cooled metal proceeds, however, due to the constant removal of the heat source at a speed from the point in question, its heating does not reach the previous maximum value. The average cooling rate of the metal in the case of the pulsed action of the heating source will be approximately determined by curve 3 drawn from the points of the minimum temperatures of thermal cycle 2. As you can see, the angle of inclination of the vector W _cool2 is much smaller, which determines a lower cooling rate of the metal, eliminates the possibility of burns and the formation of zones high stress concentrations.

The ratio of the pulsed I _{and the} and the base current I _b, the pulse frequency f, the duration of the pulse τ _u and base current τ _b and the welding speed should be chosen so as to ensure the cooling rate W _OHL heated areas of less than 12 ° C / s and to prevent lowering the temperature during the burning of the base current below the point of martensitic transformation M _n (Figure 1) for this material. The frequency of current pulses is also selected from the condition that the spots of the surface melt overlap by 20–30%, which corresponds to f = 0.1–0.3 Hz. An increase in frequency above 0.3 Hz leads to an increase in the energy introduced into the electric arc treatment zone, bringing the pulsed process closer to continuous, as a result of which the overheating zone and the volume of molten metal grow.

Lowering the frequency below 0.1 Hz increases the cooling rate during the flow of the base current, without providing the necessary overlap of the melt points. In this case, to ensure a soft thermal cycle of heat treatment, the speed of movement of the heating source V _and (arcs with non-consumable electrode) should be chosen V _and = 4 ÷ 8 m / h.

Protection of the electrode and heating sections from the influence of air is ensured by the supply of argon at a flow rate of 8 ÷ 10 l / min.

A pulsed-arc action leads to the periodicity of heating and cooling the temperature of the melt pool, which significantly affects the temperature conditions near the interface, determining the nature of crystallization of liquid metal, metallurgical and chemical processes. The pulsation frequency of the bath temperature, as well as the temperature in the solid metal near the interphase boundary, coincides with the frequency of current pulses, which directly affect the rate of formation and growth of crystals. The pulsed effect of the arc on the metal allows to achieve a refinement of the structure and increase the mechanical properties of the welded joint. At the same time, the volume of the molten metal bath when heating massive bodies is small, which eliminates the likelihood of weld deformation and the appearance of pores, and the low cooling rate of the heated sections in the temperature range of 920 ÷ 350 ° C allows you to achieve a more equiaxed structure, preventing metal quenching.

In practice, the magnitude of the base current is not more than 20 ÷ 50 A, and the duration of the flow of the base current is in the range - 2 ÷ 6 s. The minimum values of I _b from these ranges correspond to small thicknesses (less than 3 mm) of the metal being processed, the highest values correspond to thicknesses of up to 50 mm and more. If I _b > 50 A and (or) τ _b > 6 s, then an increase in the volume of the weld pool and increase in residual welding stresses is possible. When I _b <20 A, the stability of the arc and the stability of the processing process are reduced.

, определяющий допуск соотношения параметров цикла, выбирается исходя из толщины обрабатываемого металла.Coefficient

, which determines the tolerance of the ratio of cycle parameters, is selected based on the thickness of the metal being processed.

In practice, the proposed process can be implemented by using a serial welding pulse power source and a welding torch.

Example. A local argon-arc heat treatment of the welded joint was carried out with a non-consumable lanthanum electrode of the EVL brand (GOST 23949) with a diameter of ⌀3 mm at a direct and pulsed current of direct polarity. The experimental conditions and results are shown in Table 1. The highest grade argon according to TU 2114-006-5761815-99 was used as a protective gas.

The data presented in the table indicate that the proposed method, in comparison with the prototype, allows to reduce residual welding macro- and microstresses by 25 ÷ 30%, to increase the mechanical properties of the deposited metal (HAZ) due to discrete heat input and optimization of local argon-arc processing parameters .

The conditions and experimental results of argon-arc processing of HAZ welded joints of steel parts 16GS Heat Treatment Method Argon treatment mode parameters Pulse current I _u , A Properties of weld metal Stress Impact strength KCU, J / cm ² (at 20 ° C) Tensile strength σ _V , MPa Elongation δ,% 1st kind σ _I , MPa 2nd kind σ _II , MPa Prototype I _d = 160 A +110 V _St. = 15 - -65 127 21 570 eighteen m / h Proposed k = 3.3 I _b = 32 A τ _and = 2 s, +82 160 74 28 490 24 τ _b = 3 s, -fifty f = 0.2 Hz V _cv = 7 m / h

The use of this method of the heat treatment of welded joints allows to reduce the cooling rate of heated metal to values not exceeding 12 ° C / s, which eliminates the possibility of formation of unfavorable quenching structures and reduces the residual welding stresses of the first kind by 1.5–2.5 times (macrostresses) and 60–80% of the 2nd kind of stress (microstresses) stored in the metal from welding and blanking operations, evenly distributing them from the weld to the base metal. Minimization of residual welding stresses increases the resistance of the treated areas to the formation of cold cracks, hydrogen sulfide cracking and local corrosion.

The experiments showed that this method can be effectively used in the heat treatment of both welded joints of sheet materials and metal welds with a thickness of up to 100 ÷ 120 mm at a base current value of 30 ÷ 50 A.

Claims (1)

Method of argon-arc treatment of welded joints to relieve residual welding stresses, including heating by an arc of the transition section from the weld to the base metal, characterized in that the pulse-arc heating is carried out by cyclic exposure to the metal with a base current I _b = 20 ÷ 50 A and pulse current
I _and , the value of which is selected from the ratio:

, A
where τ _b is the duration of the base current, s;
τ _and - current duration in a pulse, s;
k = (1.3 ÷ 3.5) is the coefficient that determines the tolerance of the ratio of the cycle parameters, and with the frequency of the current pulses f = 0.1-0.3 Hz.

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