摘要
为研究海洋浪溅区环境下高强钢及焊缝连接的腐蚀形貌特征和时变效应,通过微观扫描测试Q690高强钢及焊缝连接表面粗糙度参数,得到表面峰最大高度(Sp)、表面谷最大深度(Sv)、表面轮廓偏斜度(Ssk)及表面轮廓峭度(Sku)随腐蚀时间的演化规律,并进行回归分析与对比.结果表明:通过分析粗糙度参数随腐蚀时间的变化过程,以及对比Q690高强钢母材与焊缝连接扫描区域的差异性,能准确地判断其腐蚀程度及特征,从而为海洋环境下国产高强钢损伤评估提供新的途径.
在“十四五”规划期间,随着中国钢铁轧制与冶炼水平的提升,以高强钢为主要材料的工程在海洋领域的建造规模取得了大幅提
迄今为止,学者们从宏观形貌分析、失重率等方面研究了普通钢材的表面损
表面粗糙度是指由波峰、波谷和波距等组成的几何形貌参数,具体可以根据微观形貌的高度分布、数据统计与极值分布,来讨论蚀坑尺寸、深度及其扩展变化规律,并以此表征腐蚀形貌规
(1) |
(2) |
(3) |
(4) |
(5) |
(6) |
式中:z(x,y)为残存表面轮廓函数;D为测定区间的坐标;Sz为表面最大高度,取Sp与Sv的代数和;Sq为表面均方根偏差值;A为测定区间的面积;M、N与lx、ly分别为扫描区域x、y方向的离散样本数量及扫描长度;p(z)为任意位置深度相关函数.

图1 腐蚀形貌示意图
Fig.1 Schematic diagram of corrosion morphology
根据GB/T 2975—1998《钢材力学性能及工艺性能试验取样规定》,试验选用武汉钢铁公司同一批次生产的板厚为10 mm的Q690低合金高强度钢材,焊条型号为CHE857Cr.首先将热机械控制工艺(TMCP)高强钢及焊接钢板按照尺寸280 mm×60 mm分别进行线切割,再按照加速腐蚀方案将其划分为6组(腐蚀时间t分别为0、20、40、60、80、100 d),每组4根平行试件.鉴于海洋浪溅区高温、干湿交替等环境特征,通过盐水周期浸润-湿热循环加速腐蚀来模拟实际海洋浪溅区环
加速腐蚀试验具体步骤为:盐水浸泡6 h、室内晾晒6 h、湿热养护12 h,以此往复循环上述操作过程.每20 d后取出1组试件,并对表面进行除锈、烘干备用,将未腐蚀试件(t=0 d)作为对照组.Q690钢材与焊条的化学组成如
Material | C | Mn | Si | P | S | Cr | Mo | Ni | V |
---|---|---|---|---|---|---|---|---|---|
CHE857Cr | ≤0.150 | ≥1.000 | ≤0.600 | ≤0.035 | ≤0.035 | ≤1.100 | ≥0.500 | — | ≥0.050 |
Q690 | 0.070 | 1.610 | 0.150 | 0.007 | 0.002 | 0.010 | 0.002 | — | 0.003 |
Diameter/mm | Current/A | Voltage/V | Welding speed/(cm·mi |
---|---|---|---|
4.0 | 130-170 | 22-26 | 15-25 |
Welding rod | Yield strength/MPa | Ultimate strength/MPa | Elongation/% | -30 ℃ impact energy/J |
---|---|---|---|---|
CHE857Cr | ≥740 | ≥830 | ≥12 | ≥27 |
试验在西安理工大学现代分析测试中心完成,测试设备采用日本OLYMPUS公司研发的激光共聚焦显微镜.首先以腐蚀试件中心为基准点(扫描尺寸为40 mm×90 mm,扫描范围为30 mm×30 mm),对其表面各微元分别进行单独扫描成像;然后通过图像拼接叠加得到整个扫描区域的微观形貌图;最后采集任意位置的表面粗糙度参数.由于微观腐蚀形貌分布存在随机

图2 腐蚀60 d试件的三维形貌图
Fig.2 3D morphology of the corroded specimen after 60 d
通过粗糙度参数统计结果,得到Q690高强钢母材及焊缝连接区扫描区域的粗糙度参数与腐蚀时间的拟合曲线,如图

图3 Q690高强钢母材的粗糙度参数
Fig.3 Roughness parameters of Q690 high strength steel

图4 Q690高强钢焊缝连接区的粗糙度参数
Fig.4 Roughness parameters of weld joint of Q690 high strength steel
由图
对比图
BCS、WCS表面Sku‑t拟合曲线始终位于Sku=3 μm上侧,说明BCS、WCS表面均分布着大量呈“峰状”的腐蚀形貌,并且表面的轮廓曲线斜率普遍较高.在加速腐蚀时BCS表面Ssk、Sku平稳增加,而WCS起伏落差较大,说明不同腐蚀阶段的WCS表面形貌变化差异相比BCS更为明显.当腐蚀60 d时,WCS表面轮廓偏斜度Ssk<0,其余周期观测的结果均为正值.而BCS表面Ssk‑t拟合曲线始终位于Ssk=0 μm下侧,说明在加速试验过程中,BCS表面主要沿深度方向发生侵蚀,产生了较多的凹陷点状腐蚀形貌.这是由于高温焊接过程的热输入,导致WCS热影响区、焊缝区微观晶粒组织及结构发生变化,经过熔焊冷却期后,连接区域存在初始残余应力,因此在应力-环境腐蚀耦合效应作用下,WCS比BCS表面更容易产生凹陷针状点蚀物,同时WCS表面分布着较多的“凸峰”状形貌,由此可知焊接区域腐蚀损伤量值偏大,表面粗糙程度十分复杂.当腐蚀100 d以后,WCS表面微观形貌沿着板厚方向的起伏落差比BCS显著.
(1)对Q690高强钢及焊缝连接表面进行微观扫描测试,获取不同腐蚀周期下的粗糙度参数(表面峰最大高度Sp、表面谷最大深度Sv、表面轮廓偏斜度Ssk及表面轮廓峭度Sku),建立了粗糙度参数与腐蚀时间的定量关系.随着腐蚀时间的增加,粗糙度参数呈幂函数增长,高强钢及焊缝连接表面腐蚀过程由不均匀腐蚀向全面腐蚀过渡,并且前者变化更加明显.
(2)在腐蚀初期,高强钢及焊缝连接表面腐蚀形态均为点蚀,焊缝连接处分布少量坑状形貌.随着腐蚀时间的增加,高强钢表面存在显著的腐蚀发展演变过程,但是在腐蚀后期时,焊缝连接表面微观形貌尺度要比高强钢更复杂.
(3)在加速腐蚀过程中,焊缝连接表面比高强钢形貌演变速率更快,最终产生更多的不规则腐蚀形貌.这是因为焊接工艺及热影响区的残余应力使得焊缝连接表面沿厚度方向的形貌起伏差异显著.
(4)采用表面粗糙度理论能够较好地反映高强钢在模拟海洋浪溅区环境下的腐蚀损伤.鉴于试验方案所选择的腐蚀周期较短,扫描试件数量偏少,因此对于Q690高强钢及焊缝连接的时变腐蚀损伤及微观形貌表征有待进一步研究.
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