Control of nitrogen content and porosity in gas tungsten arc welding of high nitrogen steel

Abstract

In welding of high nitrogen steel (HNS), it is essential to control the nitrogen content and porosity in the weld metal. In this paper, the influence of shielding gas composition and heat input on the nitrogen content and porosity in the weld metal of HNS was investigated by gas tungsten arc welding. The experimental results indicate that the weld nitrogen content increases as N₂ in the shielding gas is increased in the same heat input of welding. The weld nitrogen content decreases with increasing the heat input for pure argon used as a shielding gas, whereas it increases with increasing the heat input for the shielding gas including some nitrogen. The nitrogen pore can be avoided when the nitrogen content in the shielding gas is <4% in the heat input range of 528–2340 J mm^–1.     

Study of wind-toughness of metal arc welding with reference to multi-pass weld metal quality

The maximum cause to make mechanical toughness of a weld metal reduce in process management is known to be a mixture of nitrogen including in the atmosphere by breaking the shield condition. Mixture of the atmosphere is prevented by blowing the shielding gas such as carbon dioxide, argon, and this mixture to the arc and the molten pool in gas metal arc welding, but it is easily affected by wind. Therefore, it has been recommended conventionally that wind velocity should be controlled to less than 2.0 m/s. But it is thought that this recommendation value is unsuitable to produce multi-pass weld metal with high mechanical and porosity toughness properties because this was provided from examination results by only consideration of porosity toughness of single-pass weld metal but non-consideration mechanical toughness. In this paper, the shielding condition is evaluated not only chemical analysis and mechanical properties of multi-pass weld metal in some velocity wind environment but also visualizing varied shielding gas behaviour by the Schlieren method. As a result, it is necessary to control the wind velocity to less than 0.5 m/s to produce multi-pass weld metal with good properties. And the calculated velocity of shielding gas should be controlled to more than twice the wind velocity.     

The element transfer behavior of gas pool coupled activating TIG welding

This paper deals with a novel dual shield TIG welding method named gas pool coupled activating TIG( GPCA-TIG)welding. The welding method divides the shielding gas into two layers. Inert gas such as Ar is adopted as the inner layer gas to protect the tungsten electrode and the molten pool metal. Pure O_2,N_2 or mixture of them are used as the outer layer gas to increase the weld penetration and improve the low temperature toughness of weld metal. Through analyzing the interaction between outer gas and arc and the distributions and existing forms of oxygen and nitrogen elements,the transfer behaviors of nitrogen and oxygen from arc to pool were investigated. The results show that,the interaction between the outer gas and arc plasma makes the arc slightly constrict. The incoming oxygen enriches on the molten pool surface and exists in the form of iron oxide,chromium oxide,manganese oxide and silicon oxygen compounds. The incoming nitrogen evenly distributes in the molten pool and exists in the form of nitrogen atom.     

低镍含氮奥氏体不锈钢激光-电弧焊电弧特性及组织性能

采用100％Ar2,98％Ar+2％N2,92％Ar+8％N2,85％Ar+15％N2四种混合比例的保护气体对08Cr19MnNi3Cu2N低镍含氮奥氏体不锈钢进行激光-脉冲MAG电弧复合焊接,研究保护气体中氮气比例对焊缝中气孔数量、焊缝熔深和熔宽、电弧形态、微观组织及铁素体含量等影响机制.结果表明,随着保护气体中氮气...     

Arc ignitability,bead protection and weld shape variations for He–Ar–O2 shielded GTA welding on SUS304 stainless steel

The influences of argon and oxygen in helium base shielded GTA welding on the arc ignitability, bead protection and weld penetration are systematically investigated by bead-on-plate welding on SUS304 stainless steel. Experimental results show that the critical electrode tip work distance for arc ignition is increased from 1 mm under pure He shielding to 5 mm under He–50%Ar shielding. Small addition of oxygen content to the He–Ar mixed shielding can significantly change the weld shape from a wide shallow type to a narrow deep one, and the weld depth/width ratio can be doubled due to the change in the Marangoni convection from an outward to an inward direction.     

Microstructures of gas metal arc weld metal of 950 MPa class steel

Abstract

The effects of shielding gas composition on the properties and microstructure of single pass weld metals produced by GMA (gas metal arc) groove welding of 950 MPa class steel plates have been investigated. The shielding gas employed was a mixture of argon (Ar) and carbon dioxide (CO₂) (0–25%), and the weld heat input was ～3 kJ mm. With increasing CO₂ content, the hardness of the weld metal decreased from 380 HV to 280 HV, and the absorbed energy of the Charpy impact test decreased from 130 J to 90 J. The microstructures of the weld metal, consisting primarily of low carbon martensite and carbide free bainite, became more bainitic as the CO₂ content of the shielding gas was increased. It was also found that the MA constituent, embrittling microstructure, was formed in the granular bainitic area, the volume fraction of which increased with the CO₂ content of the shielding gas.     

低镍不锈钢激光-电弧复合焊接头组织性能研究

低镍含氮奥氏体不锈钢强度高,韧性佳,在化工装备及建筑装饰等领域广泛应用。为揭示保护气体对低镍含氮奥氏体不锈钢焊接接头微观组织和力学性能的影响机制,分别采用92%Ar+8%N2与95%Ar+5%CO2两种混合比例的保护气体对08Cr19Mn6Ni3Cu2N低镍含氮奥氏体不锈钢进行了激光-MAG电弧复合焊。研究表明：氮气的加入使焊接接头平均显微硬度有所下降;电弧收缩明显,焊接飞溅增加且体积增大,电弧稳定性变差;焊缝中奥氏体含量增加约20%,而铁素体枝晶变细,二次枝晶臂变短。焊缝组织中未发现σ相及氮化物析出;从四个晶面观察奥氏体晶粒尺寸也是由于氮气的加入而减小;焊接接头拉伸性能略微下降,但耐腐蚀性能提高。     

Effect of nitrogen addition to shielding gas on residual stress of stainless steel weldments

Abstract

The objective of the present study was to investigate the effect of nitrogen additions to the shielding gas on the ferrite content and residual stress in austenitic stainless steels. Autogenous gas tungsten arc (GTA) welding was applied on austenitic stainless steels 304 and 310 to produce a bead on plate weld. The delta ferrite content of the weld metals was measured using a Ferritscope. The residual stress in the weldments was determined using the hole drilling strain gauge method. The present results indicated that the retained delta ferrite content in type 304 stainless steel weld metals decreased rapidly as nitrogen addition to the argon shielding gas was increased. The welding residual stress increased with increasing quantity of added nitrogen in the shielding gas. It was also found that the tensile residual stress zone in austenitic stainless steel weldments was extended as the quantity of added nitrogen gas in the argon shielding gas was increased.     

超级双相不锈钢多层多道焊接接头组织及腐蚀性能

选用2507超级双相不锈钢作为研究对象,研究钨极氩弧焊多层多道焊接接头的组织和腐蚀性能.采用两种不同保护气进行钨极氩弧焊,主要讨论焊接道次和氮气添加对组织和腐蚀性能的影响.结果表明,焊缝中心均有较高的奥氏体含量,其腐蚀速率是焊根部位的0.68倍;盖面和焊根奥氏体含量相近,但盖面由于其弥散且尺寸相对较大的晶内奥氏体表现出更好的耐腐蚀性,焊根是焊缝金属的薄弱区域.混合区由于热影响区的存在腐蚀速率最快.保护气中氮气的添加促进了奥氏体的生成,降低了腐蚀电流密度一个数量级,提高了整体的腐蚀性能.     

Effects of shielding gas composition on arc characteristics and droplet transfer in tandem narrow gap GMA welding

The effects of shielding gas composition in tandem narrow gap gas metal arc welding were studied. The shielding gas included argon, carbon dioxide and helium. The arc characteristics and droplet transfer process were analysed. The results show that in the same welding parameters, the trail wire welding current is higher than the lead wire welding current. With the increase of carbon dioxide content, the welding currents of two wires decrease, and the trail wire droplet transfer mode transforms from spray transfer to projected transfer. With the increase of helium content, the welding currents increase and the lead wire droplet transfer mode transforms from projected transfer to spray transfer. The weld width is the largest when the shielding gas mixture is 80%Ar10%CO₂10%He.     

Ar-N2混合气体对奥氏体不锈钢焊缝化学成分的影响

为了验证Ar-N2混合气体对焊缝中各元素含量和焊缝铁素体数(FN值)的影响，对SS304L奥氏体不锈钢进行了4种比例的Ar-N2混合气体GTAW多层多道焊接试验，研究了4种Ar-N2比例对焊缝金属中各元素含量的影响，以及每条焊缝中各元素含量随着焊道层数的变化趋势。结果表明，不同保护气体类型的11种焊缝化学元素中，只有N元素随着保护气体中氮气比例的升高而明显增加，其它元素则没有明显的影响；经过汇总分析，认为由于稀释率的原因，导致各元素含量在同一保护气体焊缝中的规律为：C元素含量随着层数的增加而下降；Si，P，S和Nb元素含量随着层数的增加无明显上升或下降趋势；Mn，Ni，Cr，Mo和Cu元素含量随着层数的增加而上升；而N元素在纯氩气保护SG-A时的含量随着层数的增加而稍微下降，在SG-AN-0.5，SG-AN-1，SG-AN-1.5 3种保护气体中N元素含量随着层数的增加而上升；分析结果也表明,Creq/Nieq值和FN值有相同的变化趋势，都与氮气含量呈现反比关系。 创新点： 采用了阶梯式的多层多道焊，保留了各层焊缝的原始信息，验证了GTAW焊接方法在保护气体中添加氮气对奥氏体不锈钢焊缝各种化学元素成分的影响，为Ar-N2混合气体GTAW焊接奥氏体不锈钢在工程中的应用提供了可参考的试验数据。     

Effect of nitrogen on corrosion of duplex stainless steel weld metal

Abstract

A type 329Jl duplex stainless steel was gas tungsten arc welded without filler material in an Ar–N₂ gas mixture atmosphere with the aim of changing only the nitrogen content in the weld metal. The effect of nitrogen on the microstructure and corrosion properties of the weld metal was examined. An increase in nitrogen partial pressure increased the nitrogen content of the weld metal and brought reductions in the ferrite content and the quantity of Cr₂N nitride precipitates. Three corrosion parameters, namely, critical pitting temperature (CPT), pitting potential, and corrosion rate, were measured for weld metals having different nitrogen contents. The CPT and pitting potential increased and corrosion rate decreased with increasing nitrogen content of the weld metal. The corrosion behaviour was explained in terms of changes in microstructure and pitting index depending on the nitrogen content of the weld metal.     

Weld shape variation and electrode protection under Ar–(Ar–O2) double shielded GTA welding

Abstract

Double shielded gas tungsten arc welding (GTA welding or TIG welding) of an SUS304 stainless steel with pure inert argon as the inner layer shielding and the Ar–O₂ active gas as the outer layer shielding is proposed in this study in order to investigate its effect on the tungsten electrode protection and the weld shape variation. The experimental results show that the inner inert argon gas can successfully prevent the outer layer active gas from contacting and oxidising the tungsten electrode during the welding process. The active gas, oxygen, in the outer layer shielding is decomposed in the arc and dissolves in the liquid pool, which effectively adjusts the active element, oxygen, content in the weld metal. When the weld metal oxygen content is over 70 ppm, the surface tension induced Marangoni convection changes from outward into inward, and the weld shape varies from a wide shallow one to a narrow deep one. The effect of the inner layer gas flowrate on the weld bead morphology and the weld shape is investigated systematically. The results showed that when the flowrate of the inner argon shielding gas is too low, the weld bead is easily oxidised and the weld shape is wide and shallow. A heavy continuous oxide layer on the liquid pool is a barrier to the liquid pool movement.     

A computational investigation of different helium supplying methods for the improvement of GTA welding

A transient arc and weld pool model is developed, to study the effect of helium addition on the weld pool properties. Supplying mixtures of argon-helium, and alternate supply of pure argon and pure helium, are both studied.The arc characteristics are found to be highly dependent on the shielding gas composition. The addition of helium to argon increases the main governing forces in the weld pool, and more particularly the electromagnetic forces. This leads to the appearing of an electromagnetically induced vortex in the molten pool, which itself leads to an increase in the weld penetration by a factor that goes up to 3. The comparison between the numerical predictions and the experimental macrographs shows a good agreement as well as shape as dimensions.The numerical results reveal two main advantages of alternate supply of shielding gases; compared to the conventional mixtures supplying, the alternate method is more cost saving, and reduces the heat transfer to the workpiece for an equivalent weld penetration.     

氮氧联合过渡对GPCA-TIG焊焊缝的影响

针对外层引入氮氧混合气体的气体熔池耦合活性TIG焊,通过改变焊枪内外喷嘴的相对位置,分别研究了外层气体与熔池表面的耦合程度不同时焊缝成形、焊缝中氮氧含量及焊缝组织性能的变化规律.结果表明,氮氧联合过渡时气体熔池耦合活性TIG焊焊缝窄而深;低温冲击韧性高于母材及传统TIG焊的7.5%以上,而抗拉强度和屈服强度均略低于母材;焊缝组织晶粒细小,奥氏体上沿晶界分布着少量铁素体.气体熔池耦合活性TIG焊焊缝中的氮氧含量可以通过调节焊枪内外喷嘴的相对位置进行微量控制.     

Weld shape comparison with iron oxide flux and Ar–O2 shielding gas in gas tungsten arc welding

Abstract

The influence of iron oxide flux and O₂–Ar mixed shielding gas on weld shape and penetration in gas tungsten arc welding is investigated by bead-on-plate welding on SUS 304 stainless with low oxygen and low sulphur contents. The oxygen content in the weld metal is measured using a HORIBA EMGA-520 oxygen/nitrogen analyzer. The results show that both the iron oxide flux and the O₂–Ar mixed shielding gas can significantly modify the weld shape from shallow wide to deep narrow. A large weld depth/width ratio around of 0.5 is obtained when the oxygen content in the shielding gas is in the range of 3000–6000 vol. ppm. Oxygen over a certain critical value, i.e. 70 wt. ppm, in the weld pool alters the temperature coefficient of the surface tension on the pool surface, and hence changes the Marangoni convection. A thick oxide layer on the weld pool surface is generated when the oxygen content in the shielding gas is over 6000 vol. ppm, which becomes a barrier for the oxygen conveyance to the liquid pool and prevents the liquid pool from freely moving, and therefore, decreases the intensity of the Marangoni convection on the pool surface.     

保护气体对高强铝合金的焊缝组织及气孔敏感性的影响

采用普通电弧焊对2519铝合金进行焊接,研究了Ar和He二元混合气体以及Ar、He和CO2三元混合气体对接头的气孔数目、焊缝组织的影响.研究表明,采用Ar和He二元混合保护气体可以明显减少接头气孔的数目和尺寸,促进焊缝中心组织由柱状晶向等轴晶转变,且细化焊缝组织,当He气达到70%时,接头气孔数目已显著减少,焊缝中心完全呈等轴晶状态,晶粒最为细小.往30%Ar 69%He二元混合气体添加1%的CO2后接头的气孔数目进一步减少,但是焊缝组织没有明显变化.添加了He气还可以减少热影响区宽度,减弱热影响区的软化程度.     

双丝窄间隙GMAW三元保护气成分对焊缝成形的影响

窄间隙焊接在厚板焊接中有着较大的优势,保护气成分在一定程度上影响电弧行为及熔滴过渡,进而影响焊缝成形。针对30 mm厚Q235低碳钢,研究了窄间隙坡口下Ar,He和CO_2构成的三元保护气中,不同的保护气成分比例对焊缝成形的影响。结果表明:固定He含量为5%,当CO_2含量为10%时得到的焊缝表面成形良好,指状熔深相对较小,侧壁熔深相对较大的焊缝。固定CO_2含量为10%,当He含量为10%时得到的焊缝表面成形良好,指状熔深相对较小,侧壁熔深相对较大的焊缝。     

1Cr22Mn16N高氮钢激光焊接I.焊接保护气体组成和热输入对焊缝氮含量及气孔性的影响

利用CO2激光对1Cr22Mn16N高氮钢进行了焊接,研究了焊接热输入和保护气体组成对焊缝氮含量、气孔的影响。结果表明,在相同激光焊接热输入条件下,随着保护气体中氮含量的增加,高氮钢焊缝中的氮含量略有增加。当采用纯氩作为焊接保护气体时,焊缝氮含量随热输入的增加而减小;当保护气体中的氮比例达到一定比例时,焊缝氮含量随热输入的增加而增大。焊接热输入较小的条件下焊缝易产生气孔,较大的热输入将抑制焊缝中气孔的产生,而且保护气体中氮含量越高,焊缝中产生气孔的倾向越小。