根据Cr/Ni伪二元相图，基于铬镍当量比Creq/Nieq奥氏体不锈钢的凝固可分为4种模式，依次为A模式(<1.25)：L→L+γ→γ，AF模式(1.25～1.48)：L→L+γ→L+γ+(δ+γ)共晶→γ+δ，FA模式(1.48～1.95)：L→L+δ→L+δ+(δ+γ)包晶,共晶→γ+δ，以及F模式(>1.95)：L→L+δ→δ→γ+δ。冷却速度，是影响300系奥氏体不锈钢凝固行为的一个重要因素[5]。在快冷条件下一些奥氏体不锈钢的凝固由初始铁素体模式变为初始奥氏体，其热裂敏感性随之提高[6,7,8,9]。氮的加入也使奥氏体不锈钢的凝固从初始铁素体模式变为初始奥氏体，并促进铁素体转变位置处的热裂[10,11,12,13]。关于奥氏体不锈钢快速凝固微观组织和热裂已经进行了很多研究，但是尚未完全了解凝固行为变化对热裂敏感性的影响[9]。
1 实验方法
表1 301L板材的化学成分
Table 1
| C | Si | Mn | Ni | Cr | N |
|---|---|---|---|---|---|
| 0.022 | 0.32 | 1.26 | 7.32 | 17.71 | 0.13 |
表2 301L板材的力学性能
Table 2
| Plate | R0.2/MPa | Rm/MPa | δ/% |
|---|---|---|---|
| 301L-DLT | 365 | 740 | 53 |
| 301L-ST | 435 | 790 | 52 |
| 301L-MT | 524 | 880 | 37 |
| 301L-HT | 700 | 950 | 28 |
根据WRC-1992相组分图计算出301L的铬镍当量比Creq/Nieq为1.66，因此301L以FA模式凝固，铁素体含量约为4%(体积分数)。
焊接板材不开坡口，焊前将板材端面打磨后用丙酮清洗。使用Trudisk4002固体激光器进行激光焊接，光纤直径为0.5 mm，焊接功率为2.5 kW、焊接速度为1.5 m/min、离焦量为0 mm，保护Ar气0º侧吹，流量为30 L/min。
用线切割将焊件加工成拉伸试样(图1)，根据ISO 6892:1998标准在MTS材料试验机上进行拉伸实验，拉伸速度为4 mm/min。
图1
用扫描电镜和光学显微镜分析激光焊缝凝固组织，用EDS分析焊缝组织的杂质偏析，用Fischer FMP30铁磁仪测定焊缝的铁素体含量，用HXZ-1000显微硬度计测定接头硬度分布，载荷100 g，加载时间15 s。
2 实验结果
2.1 焊缝的凝固组织
图2给出了激光焊接接头的凝固组织，各局部区域微观组织的位置在焊缝整体图图2a中标出。从图2a~c可以看出，焊缝中柱状晶的凝固从熔合区两侧的板材边界起始，垂直熔合线向焊缝内生长，最终在中心线相遇。这种凝固行为使焊缝内没有中心等轴晶粒区。焊缝组织为奥氏体和枝状铁素体，焊缝底部的铁素体枝晶间距比顶部的小，底部的铁素体分布比顶部的更均匀，一次铁素体枝晶臂的平均间距约为17.5 μm。焊缝整体没有热裂纹和其它凝固缺陷，最终凝固的柱状晶接合线附近也没可见杂质，但是焊缝的顶部局部凹陷，焊缝底部的中心明显下沉。图2d~f给出了焊缝中部和底部不同区域的SEM高倍显微组织，可见铁素体呈板条状、骨架状和蠕虫状，板条铁素体束的宽度约为14 μm。铁素体的形态表明，301L激光焊缝以初始铁素体FA模式凝固，焊缝中平均铁素体量约为5.7%(体积分数)。在焊缝中心最后凝固区域的高倍组织中未见夹杂物和析出相，根据一次铁素体枝晶臂间距可推测焊缝金属熔池冷却速度约为90~100℃/s[18]。焊接热影响区的奥氏体发生了再结晶，晶粒尺寸为20~50 μm，部分晶粒明显长大。
图2
图2
激光焊接接头的显微组织
Fig.2
Microstructure of laser butt welded joint (a) overall view of welded joint; (b) optical micrograph of upper weld; (c) optical micrograph of weld bottom; (d) SEM micrograph of middle weld center; (e) lathy ferrite bunch in SEM micrograph; (f) SEM micrograph of bottom weld center
2.2 激光焊接接头的拉伸性能和断裂行为
图3给出了4种强度301L冷轧板激光焊接接头的硬度分布。可以看出，焊缝的硬度为208~241HV，平均硬度为218HV，与301L-DLT板材的硬度相近，比301L-ST、301L-MT和301L-HT冷轧板的低；三种高强度冷轧板焊接热影响区的硬度明显较低，从焊缝边缘至冷轧母材原始硬度的距离约0.5 mm。
图3
图3
不同强度激光焊接301L冷轧板的显微硬度分布
Fig.3
Microhardness profiles of laser welded 301L cold-rolled plates with different strengths
图4
图4
不同强度激光焊接301L冷轧板的拉伸曲线
Fig.4
Tensile curves of laser welded 301L cold-rolled plates with different strengths
图5给出了4种强度冷轧板激光焊接试样的拉伸断裂位置。可以看出：301L-DLT和301L-ST激光焊件的断裂位置在焊缝附近；301L-MT和301L-HT焊件的断裂位置在焊缝内，焊缝的断裂位置与焊缝顶部的凹陷位置吻合。图5还表明，301L-DLT和301L-ST拉伸试样的板材与焊缝基本上保持等截面同步变形，试样的标距发生均匀伸长和截面减小，断裂位置在板材内。这表明，两试样的拉应力尚未达到激光焊缝的断裂强度。301L-MT和301L-HT两试样断口附近的焊缝和热影响区的局部截面明显小于标距内其它部位的板材，尤其是301L-HT全硬化冷轧板试样。这表明，在焊缝及其附近区域发生了非均匀集中塑性变形，两试样的拉伸应力达到了焊缝金属的断裂强度。
图5
图5
激光对焊的4种强度301L冷轧板的拉伸断裂位置
Fig.5
Tensile fracture positions of laser butt welded 301L cold-rolled plates with four grade strengths (a) 301L-DLT, (b) 301L-ST, (c) 301L-MT, (d) 301L-HT
301L-HT板激光焊件的延伸率较低，因为板材与焊缝的硬度相差悬殊。在拉伸过程中塑性变形集中在硬度较低的焊缝及热影响区，焊接试样标距内301L-HT板的长度和截面的变化很小。301L-ST激光焊件的断裂位置不在硬度较低的焊缝，可能与试样切割损伤有关。断裂起始于两侧板材切割部位，断裂面的变形不均匀，这也是激光焊件的延伸率和断裂强度较低的原因。
图6
图6
激光焊缝和冷轧板材的断口形貌
Fig.6
Fracture morphologies of upper laser weld (a), bottom laser weld (b) and 301L-DLT plate (c)
3 讨论
在快冷条件下301L冷轧板激光焊接金属并不像与其铬镍当量比相近的304不锈钢由初始铁素体凝固模式变为初始奥氏体[3,9]，而是仍以初始铁素体FA模式凝固，并且焊缝内的铁素体量较多。对301L不锈钢激光焊接和MIG焊接微观组织的对比也证明，301L焊缝金属的凝固模式对冷却条件不敏感[15,16]。初始铁素体对热裂有重要的抑制作用，能钉扎凝固晶界增大热裂纹的扩展阻力[13]。301L焊接金属的这种快冷凝固模式，可能与其低铬、镍成分组合有关。虽然添加了奥氏体稳定元素氮，常温下其奥氏体的稳定性仍远低于304不锈钢[14]。根据高斯光束沿传播方向的线能量分布规律，焊缝熔池顶部的能量输入大于底部，即焊缝熔池顶部的温度高于底部[20]。当熔池温度降至1400 ~ 1450℃初始铁素体凝固温度区间时，焊缝顶部周围板材温度的升高和导热率的较低使顶部的冷却速度低于焊缝底部，并且顶部熔池中心和边缘的过冷度差大于底部[21,22]，因此焊缝底部的柱状晶比顶部更细、方向更一致，凝固组织的铁素体分布更均匀。焊缝中没有中心等轴晶粒区，说明焊缝金属凝固主要通过两侧的固体母材散热，焊缝纵向散热对熔池金属凝固的影响不明显。
上述结果表明，301L亚稳态不锈钢具有优异的激光加工成型性能，其初始铁素体模式凝固的激光焊缝组织保证了焊件在外载荷作用下断口内没有低延性断裂区，焊接金属的断裂强度高达886 MPa和921 MPa，远高于铬镍当量比相近的以初始奥氏体模式凝固焊接金属的断裂强度。
4 结论
(1) 301L激光焊缝以初始铁素体FA模式凝固，热裂敏感性低；焊缝柱状晶垂直熔合线向内生长在中心线相遇，没有中心等轴晶粒区。焊缝组织为奥氏体和铁素体，铁素体呈板条状、骨架状和蠕虫状，一次铁素体枝晶臂平均间距约17.5 μm，平均铁素体量5.7%体积分数。焊缝中最后凝固的中心柱状晶接合线附近没有杂质，焊缝整体没有热裂纹和析出相。
(2) 激光焊缝的硬度为208~241HV，与301L-DLT板材的硬度接近，低于301L-ST、301L-MT和301L-HT冷轧板硬度。301L-DLT和301L-ST板焊接试样的拉伸断裂位置在母材内，301L-MT和301L-HT板焊接试样的断裂位置在焊缝内，激光焊缝金属的断裂强度为886 MPa和921 MPa。
(3) 301L-HT板激光焊件的塑性低于标准规定的最低值0.2，其余三种强度冷轧板激光焊接试样的拉伸性能都达到JIS G 4305标准中相应强度冷轧301L板材的力学性能。
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