Titanium and titanium alloys are valued by people for their excellent acid corrosion resistance, and more and more are used to make equipment and process pipelines, which are applied to reactors and transportation pipelines with strong acid materials. However, titanium and titanium alloys tend to embrittle due to the reaction with various elements and compounds at high temperatures, especially during the welding process, once the oxygen, nitrogen and other elements in the air are absorbed, dissolving in titanium materials will make the alloy significant Harden, reduce the ductility of titanium metal and make it brittle, causing pipeline fracture and equipment damage. Therefore, research on the welding of titanium and titanium alloys is of great significance for the construction and maintenance of production equipment using them.
With the development of the petrochemical industry, more and more new technologies are applied to petroleum processing. At the same time, some strong acids or highly corrosive materials are used in petrochemical reactions due to their strong oxidation and good catalytic performance. But once it leaks, it will cause very serious consequences, so how to make these strong acid and corrosive materials containers and pipelines to transport them has become the focus of people’s attention. Titanium and titanium alloys are valued by people for their excellent acid corrosion resistance, and more and more are used to make equipment and process pipelines, which are applied to reactors and transportation pipelines with strong acid materials. However, titanium and titanium alloys tend to embrittle due to the reaction with various elements and compounds at high temperatures, especially during the welding process, once the oxygen, nitrogen and other elements in the air are absorbed, dissolving in titanium materials will make the alloy significant Harden, reduce the ductility of titanium metal and make it brittle, causing pipeline fracture and equipment damage. Therefore, research on the welding of titanium and titanium alloys is of great significance for the construction and maintenance of production equipment using them. This article discusses the welding quality control of titanium alloy pipes in conjunction with the installation of titanium alloy pipes in an acetic acid plant of a project.
1. Analysis of the main factors affecting the welding quality
- 1.1 The influence of high temperature heat and gas protection in the forming welding zone
At room temperature, the performance of the titanium alloy is relatively stable. At high temperatures, especially during the welding process, the titanium alloy reacts with oxygen, hydrogen, and nitrogen very quickly. According to relevant information, the temperature of the titanium alloy during the welding process is It can quickly absorb hydrogen when it is above 300 ℃, can quickly absorb oxygen when the temperature is above 600 ℃, and can quickly absorb nitrogen when the temperature is above 700 ℃, and it is most sensitive to oxidation in the air. Tests have shown that when titanium alloys are welded, if harmful gases such as oxygen and nitrogen are intruded in the molten pool, the plasticity, toughness and surface color of the welded parts have changed significantly; their mechanical properties have decreased significantly; After conducting a bending experiment on the welding, the welding meat fractured, and the base material around 10 mm around it also showed obvious slag cracking. Therefore, when welding titanium alloys, a full and reliable gas protection should be carried out on the molten pool, the droplets, and the high temperature area, whether it is the front or the back. The purpose of taking gas protection measures: one is to prevent harmful gases around the welding zone from entering the molten pool; the second is to control the high-temperature heat generation in the welding meat zone after molding. If the welding area is not effectively protected by gas, it will inevitably lead to a decline in welding quality, and in severe cases will lead to scrapping of the product.
- 1.2 Influence of impurity gases and elements
Titanium alloy welding defects are relatively common, such as pores and cracks. The causes of these defects are mainly affected by gases, elements such as hydrogen, iron, and carbon. Experiments have proved that hydrogen, iron, carbon and other gases and elements are related to the solubility of titanium alloys during welding. They are mainly derived from humid air and environment; the purity of the protective gas; moisture and oil stains on the surface of the weldment and welding wire. Among them, the cause of “stomata” is mainly from oil and impurities in the protective gas; the cause of “cracking” is mainly from moisture and oil. The occurrence of welding defects not only affects the sealing quality requirements of weldments, but also reduces the mechanical properties of joint strength, toughness, fatigue, etc., and has a greater impact on product quality. Therefore, it should be controlled when welding.
2. Control of welding quality
- 2.1 Pre-weld cleaning of weldments and consumables
The occurrence of welding defects is closely related to the cleanliness of the weldment and the surface of the welding wire. Therefore, before welding, the oil contamination, water, oxide film and other dirt on the welding wire surface should be cleaned up within 40mm. The cleaning of the edge of the tube end can be brushed with a 0.3mm thick electric or manual stainless steel wire brush to the bright metal color, and the surface of the welding wire can be polished with 80 mesh to 100 mesh diamond gauze to the bright metal color. The above treated pipe ends and welding wire surfaces should be cleaned with clean white silk cloth dipped in acetone.
- 2.2 Welding temperature and gas protection
When welding titanium pipe joints, in order to prevent the entire welding area from being contaminated by harmful gases and elements at high temperatures, it is necessary to perform the necessary welding temperature control on the welding zone and the formed welding meat. The main method of temperature control: one is to add a protective gas drag cover to the surface forming welding area; the other is to fill the welded head tube with the protective gas; the purity value of the protective gas should be equal to or greater than 99.99%; the flow rate of the protective gas should be Meet the specified value of welding technical requirements, and finally make the protective effect of the shielding gas meet the temperature control requirements of welding quality; during the welding process, timely measure the interlayer problem, use interlayer intermittent welding to ensure that the interlayer temperature does not exceed 100 ℃.
- 2.3 Welding methods and requirements
2.3.1 Reasonably select the type of welding joint, welding parameters and gas protection method according to the technical requirements;
2.3.2 Construction personnel and welders wear clean white fine gauze gloves;
2.3.3 Use a stainless steel wire brush with a wire thickness of 0.30mm, brush the butt weld to a silver-white metallic color, and then wipe the welding area with a clean white silk cloth dipped in acetone. The treated welding area is strictly forbidden to touch and touch by hand Iron items
2.3.4 Welding work should be done indoors as much as possible, the ambient wind speed should not be greater than 0.5m / s, and there should be no excessive wind;
2.3.5 Welding equipment generally selects “manual tungsten argon arc welding machine” whose performance meets the requirements of welding parameters;
2.3.6 When positioning the spot welding of the butt pipe, the butt gap is generally about 0.5mm;
2.3.7 When welding the butt joints, in order to make the quality of the weld seam meet the requirements of single-sided welding and double-sided molding, the welding seam should be welded twice: once for the back cover welding and once for the forming welding, as required Carry out corresponding gas protection;
2.3.8 The distance between the gas shield tow shield and the welding tool should be the shortest, and the gap with the pipe wall should be minimized;
2.3.9 When welding and starting the arc, the gas should be ventilated for 10s ~ 15s first. When the arc is stopped, the welding tool cannot be lifted immediately. The gas supply should continue to protect the welding area until the temperature drops below 100 ℃;
2.3.10 When welding, the welding tool should not swing left and right, and the melting end of the welding wire should not be moved out of the gas protection zone;
2.3.11 Each welding seam should be welded once as much as possible. Welding seams that must be welded should be cleaned up before welding. The length of the welding meat should be between 10mm and 15mm.
- 2.4 Qualification standards for welds and heat-affected zones
2.4.1 Weld residual height: when the wall thickness is less than 5mm, it is 0-1.5mm; when the wall thickness is more than 5mm, it is 1.0-2.0mm;
2.4.2 The weld surface must not have undercuts or tungsten inclusions;
2.4.3 The color inspection of each pass should be carried out before cleaning after welding, the qualification standards are shown in the table;
Qualification standard for color inspection of titanium bead
- 2.5 Post-weld treatment and inspection
The welding quality of titanium pipe joints must first be inspected for the surface color of the welding area, and the protection effect is above “good”. For joints with unsatisfactory protection effects, necessary quality treatment shall be carried out according to the actual situation. Under normal circumstances, when the oxidation effect is light, use a stainless steel wire brush to brush the oxidized part to the bright color of the metal (silver white) or pickle treatment to ensure that the internal metal will not continue to be oxidized. Welds that pass the visual inspection must be 100% radiographically inspected, and subsequent installation can only be carried out if they pass the inspection.
3. Control points during welding
- 3.1 Effectively control the temperature between the welding layers to avoid absorption and dissolution between the titanium alloy and nitrogen, hydrogen, oxygen gas and iron and carbon elements.
- 3.2 Use high-purity argon gas as the shielding gas for welding as much as possible, the protection effect should be good, and its flow value is within the specified range.
3.3 Welding area and the surface of the welding wire should be removed from the scale, grease and other contaminants, and the welding joint should be completed as soon as possible (within 8h).
- 3.4 Welding of on-site welds should meet the environmental requirements of titanium welding as much as possible.
Practice has proved that when welding titanium pipe joints, only good control in each link can ensure that the welding quality meets the requirements of use. With the guarantee of welding quality, titanium can be used in more special environmental conditions.