DSS经焊接热循环后，由于转变奥氏体形成导致的两相不平衡以及第二相析出，其HAZ的力学性能和耐腐蚀性能会明显降低[8,9]。对于较厚板材的焊接，单道次焊接对组织影响具有单一性，而多层多道焊接过程中后续焊道的热源对之前的HAZ仍有影响，多次的加热可导致HAZ中形成较大尺寸的晶粒组织，减小了单位体积的晶界面积，从而降低了晶界奥氏体(GBA)和魏氏体奥氏体(WA)的有效形核数量[10,11]。因此，多道次焊接热处理对GBA和WA的形成和分布具有较大影响，进而影响两相平衡，同时也可消除残余热应力，促进晶粒的再结晶和均匀化，改善HAZ的力学性能[12]。Sun等[13]发现，在2507 DSS多道次焊接过程中，第一道焊由于冷却速率快导致合金元素在两相中的分布发生变化，降低了HAZ韧性；而第二道焊热循环可通过类似固溶处理改善其韧性，但是相对较低的冷却速率易导致析出相的形成。在DSS焊接热循环过程中，奥氏体先转变为铁素体，快的冷却速率可一定程度抑制HAZ奥氏体的相变，但在一定温度范围内，双相中N的溶解度差异较大[14]，使N在铁素体中过饱和，进而促使冷却过程中Cr2N的析出[15]，降低了DSS的韧性。Ramirez等[16]研究发现，2205 DSS焊接HAZ中二次奥氏体(γ2)与Cr2N沿两相界面协同生长，易形成贫Cr区使韧性下降和塑性提升，同时，晶内奥氏体(IGA)可在氮化物上非均匀形核生长。在多道次焊接过程中，组织中最明显的变化是γ2和Cr2N的相继析出。因此，适宜的多道次焊接技术能够有效降低硬度、减少裂纹产生、细化晶粒以及调控析出相的形成，从而对HAZ的力学性能产生有益的效果[17]。
除焊接道次对HAZ的影响外，Mn、N含量变化也会影响HAZ转变奥氏体和析出相的形成，从而导致力学性能的差异。Wang等[18]研究发现，弥散分布的细小球状Cr2N析出使退火态18Cr-18Mn-0.08C-0.6N高氮奥氏体不锈钢的屈服强度提高了228 MPa。然而，在焊接过程中，N元素的损失会导致DSS的HAZ组织铁素体化及晶粒尺寸增大[19]。韩志诚等[20]发现，在2205 DSS多道焊接过程中，添加2.5%N2 (体积分数)与Ar气混合气体保护的焊接HAZ抗拉强度比单独Ar气保护更高。Yang等[21]研究表明，中锰钢焊接接头HAZ中Cr7C3的析出和晶粒粗化导致冲击韧性降低。Wang等[22]发现，在高锰奥氏体钢的模拟HAZ中，特殊晶界Σ3在裂纹扩展过程中能抑制裂纹扩展并提高裂纹扩展能。García-García等[23]的研究指出，在Fe-22Mn-1.8Al-1.2Si-0.57C 孪生诱导塑性(TWIP)钢板摩擦搅拌点焊接头中，高温热循环HAZ晶粒粗大能降低焊接接头的拉伸性能和延展性。Han等[24]和Yoo等[25]也指出，在奥氏体高锰钢焊接过程中，HAZ存在硬化区和粗晶热影响区(CGHAZ)韧性下降现象，具有潜在的热裂纹风险。Wang等[26]采用Mn和N代替Ni，使Fe-22Cr-10Mn-0.35N DSS获得更高的屈服强度并保持良好的耐腐蚀性能。Mn虽然可以提高N在DSS中的固溶度，但Mn含量对HAZ强度、塑性和析出相形成的影响仍存在争议。此外，Mn在不锈钢中还能促进σ脆性相析出，降低钢的塑韧性。与AISI304不锈钢相比，过量的Mn会导致Cr-Mn奥氏体不锈钢HAZ抗拉强度相对下降，且易发生沿晶脆性断裂[27]。
因此，目前针对双相不锈钢焊接HAZ的研究主要集中于优化焊接工艺、减少残余应力和变形，以及改变焊料成分和保护气体对材料性能的影响，缺乏Mn / N比对节Ni型双相不锈钢多道次焊接HAZ组织演变和力学性能的影响研究。本工作基于2205 DSS，采用热力学模拟试验机对4种不同Mn / N比双相不锈钢进行多道次热模拟实验，分析固溶态和焊接HAZ组织演变和力学行为，旨在通过调控Mn / N比改善节Ni型双相不锈钢的组织和力学性能。
1 实验方法
1.1 实验材料
采用VIM-20真空熔炼炉冶炼50 kg钢锭，其具体化学成分见表1，4种试样的Mn / N比分别为：3.06 (即2205钢)、3.77、17.80和65.91，对应的实验用钢编号分别为DSS1、DSS2、DSS3、DSS4。对钢锭进行锻造处理，始锻温度为1100~1200 ℃，终锻温度≥ 950 ℃，锻成宽130 mm、厚25 mm的板块，再热轧成厚度12 mm的板材，轧制温度为1100 ℃，终轧温度≥ 950 ℃，水冷。
表1 双相不锈钢试样的化学成分及Mn / N比
Table 1
| Steel No. | Mass fraction / % | Mn / N ratio | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| C | Si | Mn | S | P | Cr | Ni | Mo | Cu | N | Fe | ||
| DSS1 | 0.013 | 0.16 | 0.52 | 0.009 | 0.006 | 22.39 | 5.51 | 2.64 | 0.14 | 0.17 | Bal. | 3.06 (2205 steel) |
| DSS2 | 0.004 | 0.20 | 1.96 | 0.004 | 0.007 | 22.06 | 1.32 | 2.72 | 0.11 | 0.52 | Bal. | 3.77 |
| DSS3 | 0.011 | 0.11 | 8.01 | 0.009 | 0.007 | 22.52 | 1.37 | 2.69 | 0.14 | 0.47 | Bal. | 17.80 |
| DSS4 | 0.011 | 0.11 | 14.51 | 0.006 | 0.005 | 22.49 | 1.34 | 2.70 | 0.14 | 0.22 | Bal. | 65.91 |
1.2 焊接热模拟过程和组织分析方法
将切割后的板材经1050 ℃固溶处理30 min，以保证成分的均匀性和两相平衡，并在一定程度上消除轧制过程中形成的析出相。沿平行于轧制方向采用线切割加工成尺寸为10.5 mm × 10.5 mm × 60 mm的热模拟试样。焊接热模拟过程在Gleeble-3800热力学模拟试验机上进行，并根据Rykalin-2D公式建立模拟焊接热循环-冷却曲线方程，热输入与时间的关系服从以下方程：
式中，Q为热输入(J/cm)；t为时间(s)；T为温度(℃)；
图1
图1
DSS1~DSS4试样的多道焊接热循环曲线
Fig.1
Multi-pass welding thermal cycle curve of DSS1-DSS4 samples
1.3 力学性能测试
试样经过热模拟之后，按照ASTM E23-16b标准将其机加工成标准冲击试样，尺寸为10 mm × 10 mm × 55 mm，V型缺口位置开于热电偶处，之后用JBN-300冲击试验机在室温下进行Charpy冲击实验，再用配有能谱仪(EDS)的VEGA3型钨灯丝扫描电镜(SEM)观察冲击断口形貌并对夹杂物进行EDS分析。再将同一批次多道次焊接热模拟试样加工成标准拉伸样品，加工标准如图2所示，采用ETM205D试验机按照ASTM A370-17进行室温拉伸实验，实验结束后采用SEM分析拉伸断口形貌及夹杂物分布。最后使用DHV1000硬度计分别对热模拟后HAZ组织中的δ和γ相硬度进行测试，载荷9.8 N，保压时间15 s，每个相打3个点取平均值。
图2
2 实验结果
2.1 微观组织
图3a、c、e、g为DSS1~DSS4试样经固溶处理后显微组织的OM像。可见，DSS1试样具有典型的两相平衡的显微组织(图3a)。随着Mn / N比的提高，由于Mn、N稳定奥氏体能力的不同，DSS奥氏体由长条状转变为大片不规则形状再转变为长条状。图3b、d、f、h为多道次焊接条件下DSS1~DSS4试样HAZ微观组织的OM像。对比固溶态组织，DSS1试样的HAZ为明显二次奥氏体(γ2)组织(图3b)，主要呈现为GBA、铁素体内WA和少量小块状的IGA，且有少量黑色析出物出现在铁素体相上；DSS2试样中高温γ2主要以大量排列整齐的、相对粗大的部分转变奥氏体(PTA)形式出现(图3d)，与初始奥氏体形态高度一致；DSS3试样的焊接HAZ中形成了GBA和少量IGA，部分WA开始沿铁素体晶界从GBA中进一步生长(图3f)；DSS4试样的HAZ中WA和IGA的晶粒尺寸相对于DSS1试样晶粒较大，但是含量没有DSS1试样多，同时狭长羽毛状和部分层片状的WA从GBA向铁素体一侧生长。因此，Mn / N比变化导致析出不同的γ2相，这会对焊接HAZ力学性能产生较大影响。
图3
图3
DSS1~DSS4试样固溶态和多道次焊接热影响区(HAZ)显微组织的OM像
Fig.3
OM images of solid solution treated samples (a, c, e, g) and multi-pass welding HAZ samples (b, d, f, h) (GBA—grain boundary austenite, WA—Widmanstätten austenite, PTA—partial transformed austenite, IGA—intragranular austenite, HAZ—heat affected zone)
(a, b) DSS1 (c, d) DSS2 (e, f) DSS3 (g, h) DSS4
以奥氏体相为主要研究对象，统计了固溶态和HAZ中奥氏体相在基体中的体积分数变化。图4a为DSS1~DSS4试样固溶态和多道次焊接HAZ奥氏体总体积分数变化，图4b为HAZ各类转变奥氏体形态(GBA、WA、IGA、PTA)的占比。图4a中，增加Mn / N比使得固溶态DSS奥氏体体积分数由DSS2试样的59.21%降低到DSS4试样的56.11%，但整体均高于DSS1试样。经多道次焊接热循环后，DSS3和DSS4试样的HAZ中奥氏体体积分数分别下降至51.08%和43.68% (图4a)。结合图3和图4b可以看出，在DSS2试样中主要以体积分数58.72%的PTA为主，DSS3和DSS4试样的转变奥氏体组织主要以GBA、WA和IGA呈现，DSS3试样中形成体积分数为35.73%的GBA，DSS4试样中的GBA体积分数减少到19.16%，IGA和WA的体积分数分别增加到13.65%和10.87%，表明在焊接热循环过程中高Mn含量的添加削弱了对HAZ奥氏体的稳定性作用，促进了WA和IGA的形成。
图4
图4
DSS1~DSS4试样固溶态与多道次焊接HAZ奥氏体体积分数的变化及各奥氏体相占比
Fig.4
Variations of austenite volume fraction of solid solution treated samples and multi-pass welding HAZ samples (a) and proportions of GBA, WA, IGA, and PTA in HAZ (b)
图5
图5
DSS1和DSS2试样经多道次焊接热循环后HAZ组织的STEM像和选区电子衍射(SAED)花样
Fig.5
STEM image and SAED patterns of HAZ for DSS1 and DSS2 samples after multi-pass welding
(a-c) morphology (a), SAED pattern of γ (b), and dislocation distribution (c) of DSS1 sample
(d-i) morphology (d) and corresponding SAED patterns of γ and δ phases (e, f), dislocation distribution near γ/δ interface (g), and distribution of Cr2N precipitates (h) and corresponding SAED pattern (i) of DSS2 sample (Inset in Fig.5h is locally enlarged view)
图6为DSS3试样HAZ显微组织的STEM像和SAED花样。DSS3试样HAZ中两相分布均匀，呈条状和块状分布(图6a)，位错主要出现在δ/γ相界附近的铁素体中，并在相界出现较多位错墙(图6b)，表明位错从晶粒内向晶界生长，吸收界面的部分结合能。奥氏体内部相对光滑，在δ/γ相界处有较多胞状Cr2N析出相的生成(图6c~f)。DSS4试样HAZ中的两相差异较为明显(图7)，可观察到铁素体相中分布着高硬度的IGA和WA，导致铁素体内部发生应力集中，位错密度增加[28] (图7d和e)。通过STEM分析可知，在HAZ铁素体相内形成较多细小粒状σ相析出物(图7f~h)，尺寸仅有5~50 nm，有助于通过析出强化增加铁素体相的强度。
图6
图6
DSS3试样经多道次焊接热循环后HAZ显微组织的STEM像和SAED花样
Fig.6
STEM images and SAED patterns of HAZ for DSS3 sample after multi-pass welding
(a) distribution of GBA, IGA, and δ
(b) dislocation distribution at γ/δ interface
(c) enlarged morphology of Cr2N precipitates
(d, f) SAED patterns of ferrite (d) and austenite and Cr2N precipitates (f) in Fig.6e
(e) morphology of Cr2N precipitate at γ/δ interfaces
图7
图7
DSS4试样经多道次焊接热循环后HAZ的STEM像、SAED花样及EDS分析
Fig.7
STEM images, SAED patterns, and EDS result of HAZ for DSS4 sample after multi-pass welding
(a-c) two-phase morphology distributions (a) and SAED patterns of austenite (b) and ferrite (c)
(d) dislocation tangles near γ/δ interfaces
(e) reformed austenite morphology in two phases
(f-h) distribution of σ-phase precipitates and the amplified morphology (inset) (f), SAED pattern (g), and corresponding compositions (h)
2.2 力学性能
2.2.1 拉伸性能
图8为DSS1~DSS4试样固溶处理与多道次焊接HAZ的拉伸曲线。固溶处理后，与DSS1试样相比，DSS2试样的极限抗拉强度、屈服强度显著提高；DSS3试样的屈服平台明显增长，对应较高延伸率，但当Mn含量过高时(DSS4试样)，屈服平台有所缩短，延伸率下降。经过多道次焊接热循环后，DSS2试样HAZ的抗拉强度相比于固溶态明显降低，但断后延伸率高于固溶态，这主要由于其HAZ中PTA承担了大部分的变形，有助于抵抗拉伸引起的裂纹形成，提高了延伸率；DSS3试样的极限抗拉强度略高于DSS1试样，但延伸率明显降低；DSS4试样焊接HAZ的WA明显增多，奥氏体体积分数降低，抗拉强度明显下降。
图8
图8
DSS1~DSS4试样固溶处理与多道次焊接HAZ的拉伸曲线
Fig.8
Engineering stress-strain curves of DSS1-DSS4 samples for solid solution treated samples (a) and multi-pass welding HAZ samples (b)
图9为DSS1~DSS4试样经固溶处理与多道次焊接HAZ的抗拉强度、屈服强度和延伸率的变化。DSS2试样HAZ的抗拉强度和屈服强度分别为836和576 MPa，均高于DSS1试样，这主要是由于高N添加导致的固溶强化与沉淀强化的共同作用所致；相比于固溶处理，其HAZ的延伸率增加，表明经过多道次焊接后生成的PTA有助于抵抗热裂纹。随着Mn / N比增加至17.80 (DSS3试样)和65.91 (DSS4试样)，抗拉强度和屈服强度逐渐下降，这是由于较多高硬度的WA生成所致；DSS3试样的伸长率明显下降，这与较多GBA、WA和IGA形成，以及两相界面处的胞状Cr2N形成有关。
图9
图9
DSS1~DSS4试样固溶处理和多道次焊接HAZ的拉伸力学性能
Fig.9
Tensile mechanical properties of solid solution treated samples (a) and multi-pass welding HAZ samples (b)
为分析DSS1~DSS4试样多道次焊接HAZ的拉伸强度和塑性变化的原因，对其拉伸断口形貌作进一步分析，如图10所示。可见，DSS1~DSS4试样拉伸断口的宏观形貌主要为纤维区和剪切唇，无明显辐射区，整体表现出较高的塑性特征(图10a、d、g和j)。DSS1试样颈缩现象明显，纤维区由尺寸不一的韧窝混合而成，边缘以拉长韧窝和小而深的韧窝构成，表现出良好的塑性(图10b和c)。DSS2试样发生部分颈缩，纤维区出现明显的撕裂棱，大韧窝数量减少并且有夹杂物存在于孔洞中，为明显的韧性断裂，剪切唇主要为细小的韧窝组织，这种断裂形貌与高N含量稳定奥氏体相的作用有关，同时，PTA区析出的Cr2N相也起到了第二相强化的效果(图10e和f)。Mn / N比增加到17.80时，DSS3试样无明显颈缩，中心区域由微孔和小韧窝构成，剪切唇区域出现大量微孔集聚，表明裂纹主要沿夹杂物界面扩展，焊接HAZ中存在残余应力或者外部施加的应力与析出物共同作用，促进了晶格缺陷的累积与扩展，最终导致宏观裂纹的形成，这也是伸长率下降的主要原因(图10h和i)。DSS4试样断裂方式为准解理面和小韧窝混合而成，边缘组织由交替出现的拉长韧窝和小韧窝组成，拉长韧窝也与铁素体和夹杂物含量有关。由于Mn添加量较高，显微组织中观察到WA和IGA数量增多、尺寸增大，两相变形差增大导致塑性降低(图10k和l)。
图10
图10
DSS1~DSS4试样多道次焊接HAZ拉伸断口的SEM像
Fig.10
SEM images of tensile fractures of multi-pass welding HAZ for DSS1 (a-c), DSS2 (d-f), DSS3 (g-i), and DSS4 (j-l) samples
(a, d, g, j) marcrophologies (b, e, h, k) radial regions (c, f, i, l) shear lips
2.2.2 冲击功和显微硬度
图11a为DSS1~DSS4试样经固溶处理和多道次焊接HAZ的冲击功变化。固溶处理后，随着Mn / N比增加，DSS冲击功逐渐降低。经焊接热循环后，由于DSS1试样HAZ中WA、IGA生成并细化晶粒，HAZ的冲击功有所上升，DSS2和DSS3试样HAZ的冲击功相对固溶态下降明显，分别只有24.2和41.8 J，这与转变奥氏体引起的两相位错演变和两相硬度差变化有关。两相硬度差越大导致变形时相界处应力越集中，使DSS易发生裂纹扩展而降低冲击韧性。相对于DSS2和DSS3试样，DSS4试样HAZ的冲击功明显增加，达到134.8 J，这与10.87%体积分数的WA分割、细化的铁素体晶粒有关，有助于提高韧性。
图11
图11
DSS1~DSS4试样固溶处理和多道次焊接HAZ的冲击功及硬度分布
Fig.11
Impact energies of solid solution treated samples and multi-pass welding HAZ samples (a), and hardness distributions of the multi-pass welding HAZ samples (b)
DSS1~DSS4试样HAZ中两相的硬度结果如图11b所示。相比于DSS1试样，DSS2~DSS4试样HAZ的两相硬度差明显增大。DSS2试样HAZ的两相硬度差达到97.2 HV，随Mn / N比增加至17.80和65.91 (DSS3和DSS4试样)，GBA减少，WA和IGA不断生成，且N含量降低使得两相硬度差逐渐降低。
DSS1~DSS4试样多道次焊接HAZ冲击断口形貌的SEM像如图12所示。DSS1试样为韧性断裂，剪切唇面积最大，放射区内有明显的方块状断裂突起，阻碍裂纹的扩展，放射区为尺寸不一的韧窝的混合分布，撕裂棱明显，具有较高的韧性(图12a~c)。DSS2试样呈现出准解理断裂，剪切唇不明显，放射区出现了部分解理台阶贯穿整个断口，同时河流花样短，支流少，大颗粒夹杂物破坏了韧窝的连续性和稳定性，显著降低了HAZ的韧性(图12d和e)。DSS3试样为准解理断裂，剪切唇明显，放射区主要为脆性铁素体为主的解理断裂面，河流花样明显。这主要是由于焊接热循环后GBA的形成会在相界面产生应力集中促使位错在界面附近聚集[29]，导致两相之间位错的差异增加了变形不均匀程度。此外，解理台阶高度较DSS2试样有所降低，同时在剪切唇处观察到许多拉长韧窝，并伴有夹杂物在韧窝边缘析出(图12g~i)。DSS4试样主要表现为部分解理面和大小韧窝共存的混合断裂模式，放射区解理台阶消失，出现了形变引起的解理舌形貌，并且有大量小颗粒夹杂物随着河流花样处析出(图12k)，但剪切唇区由浅的大韧窝和密而小的韧窝混合构成，并伴有夹杂物出现，WA切割细化铁素体以及细小的IGA促进小韧窝的形成，一定程度上提高了韧性(图12l)。
图12
图12
DSS1~DSS4试样多道次焊接HAZ冲击断口的SEM像
Fig.12
SEM images of impact fractures of multi-pass welding HAZ for DSS1 (a-c), DSS2 (d-f), DSS3 (g-i), and DSS4 (j-l) samples
(a, d, g, j) marcrophologies (b, e, h, k) radial regions (c, f, i, l) shear lips
图13为DSS1~DSS4试样多道次焊接HAZ冲击断口夹杂物形貌、粒径分布及其EDS分析结果，其中粒径分布选取的是断口放射区(图12)和HAZ处的统计结果。DSS1试样HAZ断口大韧窝处出现圆形含Cr氧化夹杂物(图13a和c)，粒径主要集中在0.5~1.5 μm区间，断口和HAZ夹杂物的平均粒径分别为0.986和1.050 μm (图13b)，尺度较小不易促进裂纹的萌生，对材料韧性的影响不大。DSS2试样HAZ断口处，有较多大尺寸的球状含(Cr, Mn)O夹杂物出现在解理面和大韧窝中(图13d和f)，断口和HAZ夹杂物的平均粒径分别为2.365和2.210 μm (图13e)，其中断口大尺寸夹杂物尺寸主要集中在2.5~6.0 μm区间，HAZ中细小夹杂物尺寸聚集在0.5~1.0 μm之间，大尺寸夹杂分布均匀，尺度聚集区间与断口处相似，易产生应力集中，导致变形过程中萌生裂纹，降低了材料的韧性。当Mn / N比增加到17.80 (DSS3试样)时，HAZ断口中解理面存在尺寸不一的(Cr, Mn)O夹杂物(图13g和i)，断口和HAZ夹杂物的平均粒径分别为2.031和1.890 μm (图13h)，夹杂物尺度范围为0.5~4.5 μm，相较于DSS2试样，其夹杂物数量较少。当添加的Mn含量较高时，Mn的固溶度提高会增加(Cr, Mn)O夹杂物的形核速率和数量，导致形成较小和较分散的颗粒[30]。DSS4试样的HAZ断口也出现少量大尺寸及许多细小的球状(Cr, Mn)O夹杂物的析出(图13j和l)，断口和HAZ夹杂物的平均粒径分别为2.174和2.070 μm，断口夹杂物尺度范围为1.0~7.0 μm (图13k)，高Mn含量使得(Cr, Mn)O夹杂物尺寸最大达到7 μm，且断口和HAZ均存在1 μm左右的、较小且分散的(Cr, Mn)O夹杂物，相对于其他条件占比高且分散均匀，削弱了应力集中现象，减小了对韧性的不利影响。较低Mn / N比的DSS1试样产生尺寸较小、聚集且不易萌生裂纹的夹杂物，而较高Mn / N比的DSS4试样则增加了小颗粒夹杂物的数量和分散性，整体上平衡了少量大颗粒夹杂物带来的影响，削弱了应力集中。适中的Mn / N比(如3.77)产生较多大颗粒夹杂物，更易导致应力集中和裂纹萌生。
图13
图13
DSS1~DSS4试样多道次焊接HAZ冲击断口夹杂物形貌、粒径统计和EDS分析结果
Fig.13
Morphologies (a, d, g, j), particle size distributions (b, e, h, k), and EDS results (c, f, i, l) of inclusions in multi-pass welding HAZ impact fractures of DSS1 (a-c), DSS2 (d-f), DSS3 (g-i), and DSS4 (j-l) samples (σ—standard deviation)
3 分析讨论
3.1 Mn / N比对多道次焊接HAZ组织演变的影响
经多道次焊接热循环后DSS1~DSS4试样HAZ的显微组织演变示意图如图14所示。可见，DSS1试样(2205 DSS)由于N含量较低对析出相的影响不明显，但HAZ中GBA、WA和IGA的转变导致两相不均匀程度增加，位错的缠结也逐渐由铁素体向奥氏体发展，造成了两相位错墙的堆积。相比于DSS1试样，DSS2试样的HAZ主要为排列整齐的PTA组织，表明0.52% (质量分数)的N添加可有效稳定奥氏体，在热循环加热过程中部分转变为铁素体，导致热循环冷却后形成了接近固溶态的组织。而粗化的PTA可一定程度上钉扎铁素体，从而抑制了铁素体晶粒的长大[31]，并可有效地抑制固相线温度下Cr、Mo的偏析[32]，从而减少Cr2N的形成，少量Cr2N析出相主要在δ/γ相界处的奥氏体内形成(图5)。DSS3试样中58.72%体积分数的转变奥氏体倾向于生长在具有有限数量原子键的非理性界面(irrational interfaces)的铁素体内[33]，并且通过扩散过程从铁素体中形核和生长。HAZ组织中主要为GBA转变奥氏体组织，体积分数为6.13%的WA开始沿铁素体晶界从GBA中析出，并在铁素体内形成体积分数9.22%的块状IGA，IGA通过位错和晶内沉淀物的异质成核形成。与GBA和WA相比，IGA的形成需要更大的过冷度作为驱动力，这是因为IGA具有更高的晶格扩散激活能[34]，由于晶界扩散比晶格扩散快得多，所以IGA生长相比于GBA、WA较为受限[35]。DSS4试样HAZ中的GBA体积分数减少到19.16%，而WA和IGA转变奥氏体数量增加。同时，Cr2N析出相基本消失，这主要因为Mn的大量添加提高了N元素在钢中的溶解度，也为IGA提供了更高驱动力。Jang等[36]研究发现，当Mn含量大于2%时，Mn稳定奥氏体能力减弱，并促进σ相和高温γ2析出。因此，高Mn / N比会促进相变的进程，削弱对奥氏体的稳定能力，相比于DSS1试样，随着Mn / N比升高，焊接HAZ中奥氏体比例下降，表明N的稳定奥氏体作用远高于Mn，HAZ组织演变主要受N含量控制。
图14
图14
DSS1~DSS4试样多道次焊接HAZ中转变奥氏体演变示意图
Fig.14
Schematic of reformed austenite evolution in HAZ of DSS1-DSS4 samples after multi-pass welding
3.2 Mn / N比对HAZ拉伸性能的影响
多道次焊接对DSS1试样(2205 DSS)的HAZ拉伸性能影响较小，DSS1试样N含量低，对奥氏体的稳定性差，且两相界面处存在位错墙的堆积，少量的IGA和WA分割铁素体在一定程度上会细化晶粒，导致塑性相对于固溶态有所增加。对比DSS1试样，DSS2试样HAZ的抗拉强度、屈服强度和延伸率分别达到了836 MPa、576 MPa和39.5%，强塑性高，这归因于N大量添加导致的固溶强化和析出强化的双重作用，其中固溶强化主要是位错与基体中的间隙氮相互作用导致的基体强化，以及N在位错穿越晶格时与位错一起移动的结果[37]。同时，奥氏体中形成较多Cr2N，其带来的析出强化有效地增加了基体的硬化速率，迫使基体界面产生有效的应力/应变以适应样品的宏观变形[38]，因此，其HAZ在保持高强度的同时具有良好的塑性，对应的断口形貌为明显的撕裂棱、剪切唇和较多的细小韧窝(图10e和f)。DSS3试样固溶态的强度和塑性都高于DSS1试样，但经焊接热循环后，其HAZ的抗拉强度和屈服强度略有下降，而延伸率急速下降，这与高硬度WA的形成阻碍位错运动而导致加工硬化率增加有关，变形过程中体积分数为6.13%的WA的形成促使产生堆垛层错，最终转化为高密度位错墙(HDDWS)来协调变形[39](图6)，但其阻碍了塑性变形，强度提高而塑性降低[14]。此外，在晶界偏聚形成的胞状Cr2N也会促进拉伸裂纹的形成，降低DSS的延伸率[40,41] (图10h)。DSS4试样焊接HAZ的抗拉强度和塑性略有下降，由于Mn稳定奥氏体相的能力明显低于N，导致HAZ中转变奥氏体数量减少，奥氏体体积分数比例相对固溶态下降(图4)，从而降低了HAZ塑性。HAZ断口由拉长韧窝和部分小韧窝组成，这是因为在高Mn / N比时，较多WA、IGA的形成增加了变形过程中两相的不均匀程度，导致塑性下降。
3.3 不同Mn / N比焊接HAZ的冲击断裂机理
DSS1~DSS4试样多道次焊接HAZ的冲击断裂机理演变如图15所示。DSS1试样的多道次HAZ组织中呈现出沿δ晶界生长的WA和δ内少量IGA (图15a)，体积分数8.12%且细小的IGA与体积分数15.41%的WA分割铁素体一定程度上细化了晶粒，从而有助于提高基体的韧性，晶粒细化使晶界密度增加，部分塞积了大量位错的晶界，在进一步的冲击变形过程中容易萌生微孔洞，通过微孔聚集型机制诱发产生更为细小的韧窝[42] (图15b)，进而使DSS1试样的裂纹沿韧窝发生韧性断裂(图15c)。DSS2试样的多道次HAZ组织以PTA为主(图15d)，易引起应力集中，在PTA与铁素体晶界处位错密度显著增加，为裂纹萌生创造了条件，进而粗大的PTA在冲击变形中易形成大韧窝；大韧窝中出现的大尺寸球状(Cr, Mn)O夹杂物使得变形过程中也会产生应力集中现象(15e)，在冲击载荷作用下，裂纹沿着应力集中区迅速扩展[43] (图15f)。此外，相对于固溶态试样，冲击韧性明显降低，因为PTA产生的硬化效应使得变形区吸收冲击能量减少[44]。N大量添加使奥氏体相析出的Cr2N (图15d)与铁素体中的位错共同作用促进了脆性铁素体在相变过程中承受了比奥氏体更大的变形[45]，导致韧性降低。因此，DSS2试样主要呈现为准解理脆性断裂。DSS3试样中的裂纹主要出现在δ/γ相界处，胞状Cr2N析出相(图15g)的钉扎作用使铁素体的位错密度增加[46]，在δ/γ相界处形成大量高密度位错墙(图15h)，从而产生裂纹源，裂纹起裂成核并扩展成临界断裂面(图15i黄色圈所示)，部分形成的第二相裂纹穿过Cr2N颗粒和晶粒边界扩展成较小的解理面[9]，裂纹穿过晶粒边界扩展进入相邻晶粒形成较大的解理面(图15i)，因此，DSS3试样主要呈现以准解理断裂为主的脆性断裂。相对于DSS1试样，DSS4试样多道次HAZ组织中形成了体积分数为19.16%的更加狭长的羽毛状且硬度较高的WA和体积分数为13.65%的IGA (图15j)，它们与铁素体交叉分布，方向各异，切割细化铁素体，增加位错塞积的产生和高密度位错(图15k)，冲击变形条件下促进微孔裂纹扩展产生细小的韧窝(图15l)，但由于HAZ中奥氏体体积分数相对较低，使得冲击裂纹会穿过较粗大的铁素体晶粒导致形成脆性铁素体解理面(图15k)，且部分夹杂物也会降低韧性，但部分GBA和细小的IGA形成的小韧窝一定程度上阻止了裂纹的扩展，提高了材料韧性[47,48]。因此，DSS4试样主要呈现部分脆性解理面和大小韧窝共存的混合断裂模式。此外，Mn大量添加促进了铁素体内析出大量尺寸为5~50 nm的细小σ相(图15j)，强化了铁素体相，平衡了由于产生高硬度WA而带来的两相硬度差，有利于HAZ韧性的提高，表明高Mn / N比有利于提升多道次焊接HAZ的冲击韧性。
图15
图15
DSS1~DSS4试样多道次焊接HAZ的冲击断裂机理演变示意图
Fig.15
Schematics of impact fracture evolution of multi-pass welded HAZ of DSS1 (a-c), DSS2 (d-f), DSS3 (g-i), and DSS4 (j-l) samples
4 结论
(1) 2205 DSS多道次焊接后，HAZ中主要为体积分数24.36%的GBA并伴随大量体积分数15.41%的WA形成；相比于2205 DSS (Mn / N比为3.06)，Mn / N比为3.77时DSS的HAZ组织主要为PTA；而较高Mn / N比(17.80) DSS的HAZ组织中主要为体积分数35.73%的GBA并伴随少量体积分数6.13%的WA；高Mn / N比(65.91) DSS的GBA体积分数减少到19.16%，IGA和WA不断形成，较高Mn低N添加削弱了对奥氏体的稳定性作用，也抑制了Cr2N相的析出。
(2) 相比于2205 DSS，高N含量DSS (Mn / N比为3.77) HAZ的抗拉强度、屈服强度和延伸率分别达到836 MPa、576 MPa和39.5%，具有高的强塑性。随着Mn / N比增加，HAZ的抗拉强度逐渐降低。其中较高Mn / N比(17.80) DSS HAZ中胞状Cr2N的形成，以及相界处较多位错墙的形成导致延伸率下降。高Mn / N比(65.91)促进了WA、IGA转变奥氏体形成，导致铁素体体积分数升高，增加了变形过程中δ和γ相的不均匀程度，HAZ的塑性明显下降。
(3) N大量固溶与Cr2N析出对奥氏体相的共同强化增大了HAZ两相硬度差，且大尺寸(Cr, Mn)O夹杂物也一定程度促进了裂纹萌生，使其冲击功降至24.2 J。由于胞状Cr2N形成以及在δ/γ相界处形成的大量高密度位错墙促进了裂纹形成，较高Mn / N比(17.80) DSS主要为准解理断裂。高Mn / N比(65.91)增加了HAZ中GBA与WA形成分割细化铁素体，减少冲击裂纹萌生，形成的较多细小IGA (13.65%)阻碍了裂纹扩展，且促进了细小σ相析出，提高了铁素体硬度，平衡了两相硬度差，提高了材料的冲击韧性。
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[J].
Crystallographic understanding of the effect of Ni content on the hardenability of high-strength low-alloy steel
[J].A matrix structure with high strength, such as lath martensite/bainite is created via quenching to achieve conventional high-strength low-alloy ultra-heavy plates. Subsequently, this structure is tempered to improve its toughness. However, it is usually impossible to avoid the low cooling rate in the center of the ultra-heavy plates during cooling, causing inhomogeneous microstructure and mechanical properties along the normal direction. Therefore, it is necessary to enhance the hardenability of the alloy. At lower cooling rates, granular bainite/ferrites are formed in the center of the plates with low hardenability. While this leads to the incompletely transformed martensite/austenite islands (M/A islands), which often cause cracks, fewer high angle grain boundaries (HAGBs) are also formed, which can effectively impede crack propagation. Therefore, improving the strength, toughness, and hardenability is crucial for the development of high-strength low-alloy steel. The addition of nickel can improve the hardenability as well as the toughness of the heavy plates. In this study, two high-strength low-alloy steels with different nickel contents are designed. In addition, the effect of nickel content on hardenability and phase transition temperature is tested using end quenching and thermal mechanical simulation testing. The effects of nickel content on the microstructure and crystallographic characteristics of coherent phase-transformed products are characterized using SEM and EBSD. The results reveal that the increased nickel content greatly improves the hardenability and significantly reduces the phase transition temperature. At a low cooling rate of 0.5oC/s, the microstructure of 2.94Ni steel is lath bainite, and the M/A islands are dispersed on a thin film, forming a phase transformation mode with higher HAGB density, block boundary density and V1/V2 variant pair content, and high hardness. This mode is dominated by the close-packed plane group. While the microstructure of 0.92Ni steel is granular bainite and the M/A islands are distributed in coarse blocks, forming a phase transformation mode with lower HAGB density, block boundary density and V1/V2 variant pair content, and significantly low hardness. Moreover, this mode is dominated by the Bain group. Additionally, the results demonstrate that at the cooling rate of 0.5oC/s, as nickel content increases, the driving force of phase transformation is greatly improved to obtain a higher transformation rate than the steel with low nickel content. The maximum carbon content of untransformed austenite is higher, which promotes the complete transformation of bainite and produces fewer M/A islands. Therefore, this research possesses great potential for the composition design and process control of high-strength low-alloy steel.
Ni含量对高强度低合金钢淬透性影响的晶体学认识
[J].
Research on hot working behavior of low-nickel duplex stainless steel 2101
[J].
节Ni型2101双相不锈钢的高温热加工行为研究
[J].采用Gleeble-3800热力模拟试验机在温度为1123~1423 K、应变速率为0.001~10 s<sup>-1</sup>的条件下对2101双相不锈钢进行了热压缩实验,以研究热变形参数对其热加工行为的影响规律。结果表明,相同应变速率下,随温度升高,流变曲线由动态再结晶向动态回复转变。变形速率由0.001 s<sup>-1</sup>增至0.01和0.1 s<sup>-1</sup>提高了动态再结晶温度范围,而1和10 s<sup>-1</sup>的较高应变速率不利于动态再结晶。在应变速率为0.001~0.1s<sup>-1</sup>、变形温度为1253~1323 K时,峰值应力所对应的应变越小,奥氏体动态再结晶越容易发生,有利于等轴状再结晶组织形成。低应变速率下,变形温度升高使奥氏体再结晶晶粒长大,且Zener-Hollomon参数较大时,动态再结晶效果变差与Mn稳定奥氏体能力较Ni弱有关。基于热变形方程计算得到该不锈钢热变形激活能Q=464.49 kJ/mol,略高于2205双相不锈钢,并建立了峰值流变应力本构方程。结合不同变形条件下的应变曲线和显微组织,根据热加工图确定了最佳热加工区域为应变速率在0.001~0.1 s<sup>-1</sup>、变形温度为1220~1350 K,该区域功率耗散系数处于0.40~0.47的较高值,发生了明显奥氏体动态再结晶。
Growth kinetics of secondary austenite in the welding microstructure of a UNS S32304 duplex stainless steel
[J].
Interdiffusion kinetics of the intermetallic coatings on AZ91D magnesium alloy formed in molten salts at lower temperatures
[J].
The relationship between chromium nitride and secondary austenite precipitation in duplex stainless steels
[J].
Influence of welding thermal cycles on microstructure and pitting corrosion resistance of 2304 duplex stainless steels
[J].
An experimental and theoretical study of heat-affected zone austenite reformation in three duplex stainless steels
[J].
Evaluation of austenite reformation in duplex stainless steel weld metal using computational thermodynamics
[J].
Influence of multi-pass welding on the microstructure evolution and corrosion resistance of a super duplex stainless steel
[J].
Investigation on microstructure and impact toughness of different zones in duplex stainless steel welding joint
[J].
Influence of microstructure and elemental partitioning on pitting corrosion resistance of duplex stainless steel welding joints
[J].
Secondary austenite and chromium nitride precipitation in simulated heat affected zones of duplex stainless steels
[J].Primary and secondary intragranular austenite precipitation and its relationship with chromium nitride (Cr2N) were studied in a simulated multipass heat affected zone (HAZ) of five duplex stainless steel alloys (UNS S32304, S32205, S32550, S32750, and S32760). The Gleeble thermal-mechanical simulator was used to perform short duration and high cooling rate ferritisation and reheating heat treatments. TEM and FEG-SEM analysis, coupled with a specially developed electrolytic etching technique, revealed the cooperative growth of secondary austenite and Cr2N precipitation along the ferrite/austenite (α/γ) interfaces. Additionally, the observed close coexistence of intragranular nitride (Cr2N) and intragranular secondary austenite suggests the heterogeneous nucleation of secondary austenite from the nitrides as supported by previously reported low energy nitride/austenite (Cr2N/γ) interfaces for the observed orientation relationship between both phases. Based on these observations, a new mechanism is proposed for intragranular secondary austenite nucleation related to the intragranular nitride precipitates.
Weldability of duplex stainless steels—Thermal cycle and nitrogen effects
[J].
Enhancing yield strength of high nitrogen austenitic stainless steel
[J].
Nitrogen loss and effects on microstructure in multipass TIG welding of a super duplex stainless steel
[J].
Effect of welding process parameters on structure and property of 2205 duplex stainless steel weld joint
[J].
焊接工艺对2205双相不锈钢接头组织与性能的影响
[J].针对2205双相不锈钢的性能特点,焊接时通过优化焊接工艺以有效控制焊接热循环,使接头中能获得预期的显微组织和相比例。对获得接头进行力学性能测试,利用扫描电镜和光学显微镜观察分析其显微组织结构。结果表明:采用混合气体(氩气+2.5%氮气,体积分数)保护的钨极氩弧焊能使获得接头具有较高的强度,接头焊接区保持了与母材相同的组织结构和较为接近的相比例。
Carbide precipitates and mechanical properties of medium Mn steel joint with metal inert gas welding
[J].Medium Mn steel was metal inert gas (MIG) welded with NiCrMo-3 and 307Si filler wires. The effect of filler wires on the microstructure and mechanical properties of joint was investigated, and the carbide precipitates were contrastively discussed. The results revealed that the microstructure of weld metal, heat-affected zone and base metal are austenite. Obvious grain coarsening occurred in the heat-affected zone (HAZ), and the maximum grain size grew up to 160 μm. In HAZ, C and Cr segregated at grain boundaries, the carbides was identified as Cr7C3. The dispersive (Nb, Mo)C phase was also found in weld metal with NiCrMo-3 filler wire. All the welded joints failed in HAZ during tensile tests. The tensile strength of welded joint with NiCrMo-3 filler wire was 675 MPa, which is much higher than that with 307Si filler wire. In comparison to base metal, higher microhardness and lower impact toughness were obtained in HAZ for these two welded joints, which was attributed to the precipitation of Cr7C3 phase and grain coarsening. The impact toughness around the fusion line is the worst for these two welded joints.
Superior cryogenic toughness of high-Mn austenitic steel by welding thermal cycles: The role of grain boundary evolution
[J].
Experimental and FEM study of Ti-containing TWIP steel weldability
[J].
Hot ductility and hot cracking susceptibility of Ti-modified austenitic high Mn steel weld HAZ
[J].
Microstructural evolution and solidification cracking susceptibility of Fe-18Mn-0.6C-xAl steel welds
[J].
Nickel-free duplex stainless steels
[J].
Effect of welding passes on heat affected zone and tensile properties of AISI 304 stainless steel and chrome-manganese austenitic stainless steel
[J].
The heterogeneity of persistent slip band nucleation and evolution in metals at the micrometer scale
[J].\n Metal fatigues when repeatedly loaded, ultimately failing when cracks form and propagate through the material. Lavenstein\n et al.\n studied the origins of this process in nickel. Using high-resolution observations, they tracked how dislocations evolved into microstructural features called persistent slip bands that preceded crack formation. The evolution of tangles of dislocations to a more regularly spaced pattern form the basis for the persistent slip bands and provide a road map for understanding fatigue cracking in metals.\n
Evolution of phase ratio and its effect on residual stress for 2205 duplex stainless steel multipass welded joints by thermo-metallurgical-mechanical model
[J].
Formation, evolution and remove behavior of manganese-containing inclusions in medium/high manganese steels
[J].
Microstructure and mechanical property relationship for different heat treatment and hydrogen level in multi-pass welded P91 steel joint
[J].
Microstructural characterization of simulated heat affected zone in a nitrogen-containing 2205 duplex stainless steel
[J].
Effect of root pass filler metal on microstructure and mechanical properties in the multi-pass welding of duplex stainless steels
[J].
Effect of current type on microstructure and corrosion resistance of super duplex stainless steel claddings produced by the gas tungsten arc welding process
[J].
New insights into the character of austenite-ferrite boundaries in an additively manufactured duplex stainless steel
[J].
Effect of different Mn contents on tensile and corrosion behavior of CD4MCU cast duplex stainless steels
[J].
Novel high nitrogen austenitic stainless steels: From high-throughput screening to experimental validation and properties relationship
[J].
Effect of Mn / N ratio on microstructure and mechanical behavior of simulated welding heat affected zone in 22% Cr lean duplex stainless steel
[J].
Mechanical properties of nanostructured 316LVM stainless steel annealed under pressure
[J].
Satyanarayana. Effect of high temperature ageing on microstructure and mechanical properties of a nickel-free high nitrogen austenitic stainless steel
[J].
Time-temperature-precipitation characteristics of high-nitrogen austenitic Fe-18Cr-18Mn-2Mo-0.9N steel
[J].
Effects of ultrafine grains on fracture toughness of nanocrystalline materials
[J].
超细晶粒对纳晶材料断裂韧性的影响
[J].
Effect of tensile overload on fatigue crack behavior of 2205 duplex stainless steel: Experiment and finite element simulation
[J].
The strengthening role of post-welded cryogenic treatment on the performance and microstructure of 304 austenitic stainless steel weldments
[J].
Microstructure and pitting corrosion of shielded metal arc welded high nitrogen stainless steel
[J].The present work is aimed at studying the microstructure and pitting corrosion behaviour of shielded metal arc welded high nitrogen steel made of Cromang-N electrode. Basis for selecting this electrode is to increase the solubility of nitrogen in weld metal due to high chromium and manganese content. Microscopic studies were carried out using optical microscopy (OM) and field emission scanning electron microscopy (FESEM). Energy back scattered diffraction (EBSD) method was used to determine the phase analysis, grain size and orientation image mapping. Potentio-dynamic polarization testing was carried out to study the pitting corrosion resistance in aerated 3.5% NaCl environment using a GillAC electrochemical system. The investigation results showed that the selected Cr–Mn–N type electrode resulted in a maximum reduction in delta-ferrite and improvement in pitting corrosion resistance of the weld zone was attributed to the coarse austenite grains owing to the reduction in active sites of the austenite/delta ferrite interface and the decrease in galvanic interaction between austenite and delta-ferrite. © 2015 China Ordnance Society
The influence of chromium content on the precipitation of γ' (ordered NI3Ti) in some austenitic steels
[J].
Micromechanism of cleavage fracture of weld metals
[J].Cleavage fracture is the most dangerous form of fracture. Cleavage fracture usually happens well before general yielding at low nominal fracture stress and strain. Cleavage fracture is often spurred by low temperature and determines the toughness in the lower shelf temperature region. This paper describes a new framework for the micromechanism of cleavage fracture of high strength low alloy (HSLA) steel weld metals. Cleavage fracture not only determines the impact toughness in the lower shelf but also plays a decisive role on the impact toughness in the transition temperature region. The toughness is determined by the extending length of a preceding fibrous crack which is terminated by cleavage fracture. Three non-stop successive stages, i.e. crack nucleation, propagation of a second phase particle-sized crack across the particle/grain boundary, propagation of a grain-sized crack across the grain/grain boundary are explained. The "critical event" of cleavage fracture is emphasized which offers the greatest difficulty during crack formation and controls the cleavage process. The critical event indicates the weakest microstructural component and its critical size which specifies the local cleavage fracture stress σf for cleavage fracture. In toughness-study it is paramount important to reveal the critical events for various test specimens. Three criteria for crack nucleation, for preventing crack nucleus from blunting and for crack propagation are testified. An active region specified by these criteria is suggested where the combined stress and strain are sufficient to trigger the cleavage fracture. It can be used in statistical analyses. A case study, using the new framework of micromechanism for analyzing toughness of 8%Ni steel welding metals is presented to analyze the experimental results.
焊缝金属解理断裂微观机理
[J].本文为低合金高强钢的解理断裂提出了一个新的理论框架：解理断裂不仅决定下平台的冲击韧性,而且对过渡温度区的韧性也起决定性的作用,因为在这个温度区间韧性取决于先前产生的塑性裂纹扩展的长度,而解理断裂终止了塑性裂纹的扩展,从而决定其长度。解理断裂包含3个不间断的阶段：(1) 裂纹在第二相颗粒起裂成核;(2) 第二相裂纹穿过第二相颗粒和晶粒边界扩展;(3) 晶粒裂纹穿过晶粒边界扩展进入相邻晶粒。本理论框架对整个过程进行了诠释：解理断裂的临界事件是在裂纹形成过程中提供最大困难的阶段,它控制了解理断裂过程,并确认了最薄弱的微观组分及其临界尺寸,这个尺寸决定了微观解理断裂应力σ<sub>f</sub>。在断裂过程研究中最为重要的就是揭示各种条件下断裂的临界事件。提出引发解理断裂的3个准则：(1) 裂纹起裂成核准则;(2) 防止裂纹核钝化准则;(3) 裂纹扩展准则。并由这3个准则形成了一个活性区,在活性区中应力和应变的综合作用可以引发解理断裂,这个活性区被用来建立断裂的统计模型。本研究利用这个新的理论框架进行了8%Ni钢焊缝的实例研究,以说明实验研究的结果。
