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.     

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.     

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.     

冷却速率对高氮钢焊缝组织和性能的影响

研究了水冷和空冷条件下高氮不锈钢焊缝金属微观组织和力学性能的变化规律,讨论了冷却速率对高氮不锈钢焊缝微观组织和力学性能的影响规律. 结果表明,冷却速率增加能够有效增加高氮钢焊缝金属中的氮含量,尤其对于含氮量0.85%的高氮含量焊丝,增氮效果更明显. 冷却速率增加对高氮钢焊缝金属抗拉强度提高程度取决于焊丝中的氮含量,对于低氮含量高氮钢焊丝,冷却速率增加能够显著提高焊缝金属抗拉强度,当焊丝中氮含量超过0.58%时,冷却速率增加对焊缝金属抗拉强度影响不大,最终接头强度达到850 MPa.     

Q550高强钢焊接接头强韧性匹配

在不预热条件下采用不同合金成分焊丝焊接Q550高强钢,试验研究焊丝中合金对焊缝组织、接头抗拉强度及冲击韧性的影响.结果表明,使用MK.G60-1焊丝可获得以针状铁素体为主的焊缝组织.焊缝中沿晶界分布的先共析铁素体在承受拉应力时易萌生裂纹,提高焊缝中针状铁素体含量可以提高接头抗拉强度和韧性.采用MK.G60-1焊丝接头抗拉强度接近母材的抗拉强度,断裂发生在熔合区.接头热影响区的冲击吸收功最高,而熔合区的抗拉强度和韧性最低.焊缝冲击断口纤维区均以穿晶断裂为主,断口韧窝产生的机理是微孔聚集型,针状铁素体区对应的韧窝较大,先共析铁素体对应的韧窝较小.     

Friction stir welding of DH36 steel

Abstract

Hot rolled DH36 carbon steel, 6.4 mm in thickness, was friction stir welded at speeds of 3.4 mm s^-1 (8 in min^-1), 5.1 mm s^-1 (12 in min^-1), and 7.6 mm s^-1 (18 in min^-1). Single pass welds free of volumetric defects were produced at each speed. The relationships between welding parameters and weld properties are discussed. Optical microscopy, microhardness testing, and transverse and longitudinal tensile tests have been performed. Bainite and martensite are found in the nugget region of the friction stir welds whereas the base material is comprised of ferrite and pearlite. The maximum hardness is observed in the weld nugget, and the hardness decreases gradually from the weld nugget, through the heat affected zone, to the base metal. Tensile testing also indicates overmatching of the weld metal relative to the base metal. Maximum hardness and longitudinal (all weld metal) tensile strengths increase with increasing welding speeds. Weld transverse tensile strengths are governed by the base metal properties, as all transverse tensile bars fail in the base metal.     

Effect of the cooling rate on the microstructure and mechanical properties of high nitrogen stainless steel weld metals

The microstructure and mechanical properties of high nitrogen steel(HNS) weld metals prepared under air-and water-cooling conditions are investigated, and the effect of the cooling rate on these properties is discussed. The results indicate that an increase in the cooling rate could significantly increase the nitrogen content in HNS weld metals, especially for weld metals with a nitrogen content of 0.85%.Moreover, increasing the cooling rate could result in an increase in the tensile strength of HNS weld metals, which is found to be strongly dependent on the nitrogen content of the HNS sample. For high nitrogen austenitic stainless steel welding wire with lower nitrogen content, increasing the cooling rate could significantly improve its tensile strength, but a higher cooling rate has no influence on weld metals with nitrogen content less than 0.58%. The tensile strength of the joint reached 850 MPa.     

Investigation of the mechanical properties of welded joints between corrosion-resisting steel and titanium alloys

Summary

The purpose of this paper is to investigate the impact properties of weld metal produced under different welding conditions with special reference to electroslag welding (ESW) as a high heat input welding process generally applied in the fabrication of four‐sided thick‐plate box columns for general multi‐storey buildings. The paper focuses in particular on the impact properties of ESW weld metal at its centre (core (C)) and periphery (rim (R)). The results obtained may be summarised as follows:

1. The vE value of the weld metal core is lower than that of the weld metal rim. The absorbed energy transition temperature is also higher.

2. The vE values of the weld metal core and rim altered with a change in the welding heat input Q (varied at six levels in the 10.1 ～ 126.7 kJ/mm range), generally decreasing with an increasing heat input. The vE value of the core is lower than that of the rim. In the weld metal produced at the maximum heat input (126.7 kJ/mm), however, the core and rim have much the same vE values.

3. The vE value of the weld metal core shows little change with a change in the steel type (one type of TMCP and two types of SM490A), and remains at a low value. When there is a change in the surrounding gas (oxygen (O₂), air, and argon gas), the vE value decreases in an Ar, air, O₂ sequence.

4. The weld metal core vE value is little dependent on any change in the flux type (three types of fused flux) and weight of flux used (0.5 or 0.7 N).

5. The microstructural observations suggest that grain‐boundary ferrite (GBF) occurs at the coarse prior austenite grain boundaries of the rim, with fine acicular ferrite (AF) being mainly found trans‐granularly. In the core, grain‐boundary ferrite is formed at high density at the fine prior austenite grain boundaries, with massive polygonal ferrite (PF) being formed at high density transgranularly. There are thus distinct differences in the coarse ferrite morphology of the rim and core.

6. The SEM observations of the fracture surfaces suggest that the core weld metal has a large grain size, a feature corresponding to its low vE value. The observations made at the fracture surface periphery indicate that numerous secondary cracks are initiated in the grain‐boundary ferrite of the core, suggesting that the ferrite has a low toughness.

7. The results of the SEM simultaneous fractographic‐microstructural observations suggest that fracture selectively propagates along the grain‐boundary ferrite. This indicates that the low vE value of the core is due to the presence of high‐density grain‐boundary ferrite and massive transgranular polygonal ferrite.

     

Effect of titanium addition on the microstructure and inclusion formation in submerged arc welded HSLA pipeline steel

The effect of titanium addition on the SAW weld metal microstructure of API 5L-X70 pipeline steel was investigated. The relationship between microstructure and toughness of the weld deposit was studied by means of full metallographic, longitudinal tensile, Charpy-V notch and HIC tests on the specimens cut transversely to the weld beads. The best combination of microstructure and impact properties was obtained in the range of 0.02–0.05% titanium. By further increasing of titanium content, the microstructure was changed from a mixture of acicular ferrite, grain-boundary ferrite and Widmanstätten ferrite to a mixture of acicular ferrite, grain-boundary ferrite, bainite and ferrite with M/A microconstituent. Therefore, the mode of fracture also changed from dimpled ductile to quasi-cleavage. The results showed an increase in the titanium content of inclusions with increased titanium levels of weld metal. Titanium-base inclusions improve impact toughness by increasing the formation of acicular ferrite in the microstructure. No HIC susceptibility was found in the weld metals with titanium contents less than 0.09%.     

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.     

Characteristics of Nd： YAG laser welded 600 MPa grade TRIP steel

Microstructures and mechanical properties of Nd: YAG laser welded transformation induced plasticity (TRIP) steel with tensile strength of 645 MPa were studied. Due to high cooling speed of laser welding, the weld metal mainly consists of martensite different from the base metal, which is composed of ferrite matrix with bainite and a little retained austenite. Therefore, the weld metal has maximum hardness at welded joint. The yield strength and tensile strength of welded specimens tested perpendicular to weld line were almost equal to those of the base metal. But the yield strength and tensile strength of welded specimens tested parallel with weld line were a little higher than those of the base metal. The formability of laser welded TRIP steel was decreased compared with that of the base metal.     

Effect of normalising heat treatment on microstructure and properties of nickel alloyed C-Mn weld metals

Abstract

Five different basic manual metal arc welding electrodes, containing varying amounts of nickel (from 0 to 3.5%) were deposited in an all weld metal joint. Mechanical testing and microstructure examination was performed in the as deposited and heat treated conditions. The heat treatment was carried out at three different temperatures (930, 980, and 1030° C) for 20 min. The tensile strength was decreased by the heat treatment, but the magnitude of the decrease varied between the weld metals. The impact properties were also affected by the heat treatment. For impact properties, however, a decrease was found at low testing temperatures, whereas an increase was observed at higher testing temperatures. The decrease in tensile strength after normalisation, compared with the as deposited condition, is due to an increasing grain size and a decreasing dislocation content. The strength achieved by the different weld metals in the normalised condition can be explained by the variation in solid solution hardening resulting from differences in the alloying content.Two factors seemed to be especially important in determining the variations in impact properties between weld metals in the as deposited condition. The nitrogen content of the weld metals decreased the impact toughness, whereas increasing nickel content was associated with improved impact toughness. In the normalised condition, reduced at lower testing temperatures, because cleavage fracture started readily in the resulting coarser grains. Furthermore, traces of segregated bands of microphases probably acted as initiation sites for cleavage cracks. At higher testing temperatures, higher impact toughness was obtained, owing to the lower strength of the weld metals. One of the electrodes showed superior impact toughness values to the other electrodes, in both the as deposited and heat treated conditions. The main reason for the high toughness in the as deposited condition was the ability of this electrode to refine previously deposited beads to a high degree. The reason for the high toughness after normalising is still not certain, but it was noted that this weld metal had a very low oxygen content and also a comparatively low volume fraction of segregated microphases. These factors might be important in achieving the very high impact toughness observed.     

外层氧气引入对GPCA-TIG焊焊缝性能的影响

针对SUS304不锈钢,采用传统TIG焊和GPCA-TIG焊进行工件表面熔焊,对外层气体引入氧气的GPCA-TIG焊和传统TIG焊焊缝的氧含量、显微组织、拉伸性能和低温冲击韧性进行了测定. 结果表明,GPCA-TIG焊焊缝组织主要为奥氏体和铁素体,铁素体形态以骨架状和板条状为主. 外层引入氧气时,焊缝中的氧含量增加,耦合度为+2时焊缝中的氧含量高于耦合度为0时的,焊缝的抗拉强度均略低于母材的. 耦合度为0的GPCA-TIG焊焊缝冲击性能与传统TIG焊的相同,耦合度为+2的焊缝低温冲击韧性有所降低,达到传统TIG焊的85%.     

700MPa级低合金高强钢低匹配焊接接头组织和性能研究

按照低匹配设计原则,采用熔化极气体保护焊对屈服强度为700 MPa级的低合金高强钢进行了焊接。采用光学显微镜、扫描电镜和力学性能试验研究了焊接接头的组织和性能。结果表明,焊缝区组织主要为针状铁素体和少量先共析铁素体,粗晶热影响区组织为上贝氏体;焊缝区显微硬度明显低于母材,焊缝区-40℃时的冲击韧度(AKV)为58 J,抗拉强度达到了母材抗拉强度的92.4%,焊接接头的综合力学性能良好。     

气体保护焊保护效果研究综述——氮含量对气孔及焊缝性能的影响

研究了气体保护焊时氮含量对气孔及力学性能的影响,试验结果表明:采用实心焊丝和ψ(Ar)20％ψ(C02)80％组合时,随着环境中氮含量的增加最容易产生气孔.因此,必须把保护气体中氮的含量控制在1％以下.采用药芯焊丝和C02组合时,耐气孔性良好,但是,随着氮含量的增加冲击吸收功下降.因此,应把熔敷金属中氮的含量限制在0.01％以内.     

管线钢大电流双面高速埋弧焊接用焊丝研制

介绍了石油、天然气输送管线用埋弧焊丝的特征及高韧性焊丝成分设计原则,分别采用92kg高频感应炉实验室冶炼。500kg中频感应炉半工业性冶炼和300t顶吹氧气转炉工业化大生产冶炼、进行焊丝研制。对所研制的焊丝配合烧结焊剂SJ101,进行了无坡口双面埋弧焊试验,测试了焊缝成分、冲击韧度、金相组织、硬度和接头抗拉强度,用扫描电镜分析了冲击断口形貌及夹杂物组成,透射电镜分析了焊缝金属的微观结构,俄歇试验分析了B、Ti、N在焊缝中的分布。结果表明,B在原奥氏体晶界偏聚,可抑制先共析铁素体在晶界析出,弥散分布的细小夹杂物有助于针状铁素体的生成,配合降低S、P、气体元素和杂质含量,可使焊缝金属具有高的韧性,焊缝硬度在HV185～HV214之间,与母材平均硬度HV190为同一水平。焊缝金属横向抗拉强度高于母材。     

B元素对药芯焊丝焊缝金属针状铁素体形成的影响

利用金相组织观察、冲击试验和热膨胀试验,研究了B元素含量变化对高强钢药芯焊丝焊缝金属中针状铁素体形成的影响,得到了不同试验温度下焊缝金属冲击吸收功. 结合透射电镜分析和分级淬火试验从热力学和动力学的角度对B元素影响机理进行了分析. 结果表明,焊缝金属组织晶界中含有自由状态的B元素具有抑制晶界铁素体形核利于针状铁素体生成的作用;N元素含量增加会降低晶界B元素含量,并提高奥氏体向铁素体转变的温度,减少针状铁素体含量;针状铁素体是在以Ti元素和Mn元素的氧化物为核心,以Cu元素和Mn元素的硫化物为外层,以BN为过渡层的复杂结构上形核并长大的;针状铁素体含量的增加有利于提高焊缝金属冲击吸收功,–60 ℃冲击吸收功最大为70 J.     

Mechanical properties of joints welded in creep-resistant low-alloy T24 steel

Summary

During TIG welding of Zircaloy‐2 tubing in an atmosphere in which the air partial pressure P_AIR decreases as high‐purity inert gases are introduced into a chamber evacuated to various pressures, the arc voltages were measured, and the properties of the welds were examined.

The arc voltage increases with a decreasing P_AIR in the welding atmosphere.

The nitrogen content [N] in the weld metal varies in each circumferentially divided zone, with a linear relationship between √SRP_NZ and [N] being found.

The thickness of the surface oxide films after long‐term autoclave tests in water at high temperature and pressure was also measured and evaluated.     

Impact toughness of 316 stainless steel weld metal on elevated temperatures aging

Abstract

Shielded metal arc welding electrodes of a modified E316-15 austenitic stainless steel, for service at 673–823 K with delta ferrite in the range of 3–7 ferrite number, have been developed indigenously for welding of 316L(N) stainless steel structural materials for the Indian Prototype Fast Breeder Reactor. Delta ferrite content in weld metals for high temperature service is restricted for limiting the formation of embrittling secondary phases during service. To study the effect of high temperature exposure on microstructure and mechanical properties, the 316 weld metal was aged at three different temperatures of 923, 973 and 1023 K, for various durations up to 500 h. The activation energy for the transformation of delta ferrite has been estimated to analyse the mechanism associated with the micro structural changes that led to the deterioration in toughness on elevated temperature aging of this weld metal.     

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.