Created on:2021-07-29 09:16

Hydraulic failure of opening and closing of hydropower equipment_ Abnormal failure of chromium plating on piston rod

水电设备启闭液压故障_ 活塞杆镀铬层非正常失效
Hydraulic failure of opening and closing of hydropower equipment_ Abnormal failure of chromium plating on piston rod

    此处的非正常失效主要是指活塞杆在使用一年内即产生锈蚀及镀铬层的破坏。
The abnormal failure here mainly refers to the corrosion of piston rod and the damage of chromium plating layer within one year.

(1)镀铬层的正常失效
(1) Normal failure of chromium plating

①由针孔及孔隙造成的锈蚀 镀双层铬(先镀乳白铬后镀硬铬)不可避免地会出现孔隙,使用时,水气通过针孔及孔隙到达母材,时间长了就逐渐锈蚀,锈蚀面积大了、严重了就进,步造成镀铬层剥落,这种失效在褪镀后蚀坑边缘是圆滑的。
① The corrosion caused by pinholes and pores, double chromium plating (first milky white chromium plating, then hard chromium plating) will inevitably appear pores, when in use, water vapor through pinholes and pores to the base metal, a long time on the gradual corrosion, corrosion area is large, serious step by step, resulting in stripping of the chromium plating layer, this failure after stripping of the pit edge is smooth.

②磨损造成的镀层减薄 当镀层全部被磨损就会产生锈蚀。
② When the coating is worn completely, corrosion will occur.

(2)镀铬基的几种非正常失效
(2) Several abnormal failures of chromium plating base

    ①锈蚀部位在褪镀后蚀坑边缘是非圆形的 (有折角),出现裂纹或麻丝状,其尾部是尖的。这是比较典型的由内部应力造成的失效。
① After deplating, the edge of the corrosion pit is non-circular (with corner), with cracks or hemp filaments, and its tail is sharp. This is a typical failure caused by internal stress.

②活塞杆涂有油脂的外伸部位在油脂层未损坏情况下不到一个月的时间就锈蚀了。
② The extended part of piston rod coated with grease was corroded in less than one month without damage of grease layer.

    ③在对返修的活塞秆进行褪镀前检测时,用蓝点法(贴滤纸法)测试孔隙未测出蓝点,而褪镀后发现该处有裂纹或蚀坑。
③ During the inspection of the repaired piston rod before stripping plating, the blue dot method (filter paper sticking method) was used to test the pores, and no blue dot was detected, but cracks or corrosion pits were found after stripping plating.

④褪镀后经加工的表面还有疏松,有的经油浸后留有油迹(擦不掉但用砂布能磨掉)。凡留有油迹的地方必有灰点等缺陷。
④ After stripping, the processed surface is still loose, and some oil stains are left after being immersed in oil (which can not be wiped off but can be worn off with abrasive cloth). Where there are traces of oil, there must be grey spots and other defects.

(3)镀铬层非正常失效的原因分析
(3) Cause analysis of abnormal failure of chromium plating layer

    ①未进行镀前消应和镀后去氢是非正常失效的原因之一 GB 11379—89(《金属覆盖层工程用铬电镀层》)及GB/T 12611-90《<金属零件镀覆前质量控制技术要求》)规定凡钢件的抗拉强度大于1050MPa的都要在镀前消除加工应力并在镀后去氢,因此,专家认为凡是抗拉强度达到800MPa(已属于高强钢)的就要去氢。对40Cr活塞杆只规定抗拉强度大予630MPa而没规定上限是不够的,至少要限制上限在800MPa以下。非正常失效的活塞杆抗拉强度多数达到800~900MPa,有的已超过900MPa。对于大件(可认为直径大于300mm,长度大于12m的属于大件)尽管抗拉强度未达到800MPa,专家认为也应进行镀前消应、镀后去氢。导流底孔活塞杆因为镀前采取停置(7d)时效消应和镀后去氢才未锈蚀(泄洪深孔活塞杆却未进行镀前消应和镀后去氢)。
① It is one of the reasons for abnormal failure that stress relief before plating and dehydrogenation after plating are not carried out. According to GB 11379-89 (metallic coatings, chromium electroplating for engineering purposes) and GB / T 12611-90 (technical requirements for quality control before plating of metallic parts), the processing stress of steel parts with tensile strength greater than 1050mpa should be eliminated before plating and dehydrogenation after plating, Experts believe that those whose tensile strength reaches 800MPa (already belongs to high strength steel) should be dehydrogenated. For 40Cr piston rod, it is not enough to specify 630mpa higher tensile strength without upper limit, which should be limited below 800MPa at least. The tensile strength of piston rod with abnormal failure mostly reaches 800-900 MPa, and some have exceeded 900 MPa. For large parts (it can be considered that the diameter is greater than 300 mm and the length is greater than 12 m) even though the tensile strength does not reach 800 MPa, experts believe that stress relief before plating and dehydrogenation after plating should also be carried out. The piston rod of diversion bottom hole was not corroded because it was stopped (7D) before plating for stress relief and dehydrogenation after plating (the piston rod of flood discharge deep hole was not corroded before plating for stress relief and dehydrogenation after plating).

    关于镀前不消应及镀后不去氢的坏处分析如下。镀铬时20%的电流用在镀铬上,还有20%的电流用在还原六价铬上,而其余60%的电流用于析氢。电镀时、必然会析出镀液中的氢,析出的氢一部分进入大气,也有一部分进入母材中,如果未及时地将进入到母材中的氢驱除掉,就会在以后的加工过程中或安装及使用中产生氢脆裂纹,这些裂纹将破坏镀层的结合力,造成镀层剥落。因此,去氢应在镀后3h内及时进行。电镀前,机械加工会使像40Cr这样的对应力敏感的材质产生加工应力,故也要进行消应处理,有应力存在就可能随时释放出来影响镀铬质量。
The disadvantages of no stress before plating and no dehydrogenation after plating are analyzed as follows. During chromium plating, 20% of the current is used for chromium plating, another 20% is used for reducing hexavalent chromium, and the remaining 60% is used for hydrogen evolution. During electroplating, hydrogen in the plating solution will inevitably precipitate. Part of the precipitated hydrogen will enter into the atmosphere, and some will enter into the base metal. If the hydrogen in the base metal is not removed in time, hydrogen embrittlement cracks will occur in the later processing or installation and use. These cracks will destroy the adhesion of the coating and cause the coating peeling off. Therefore, dehydrogenation should be carried out within 3 hours after plating. Before electroplating, machining will cause processing stress in stress sensitive materials such as 40Cr, so it is necessary to carry out stress relief treatment. If there is stress, it may be released at any time, affecting the quality of chromium plating.

②活塞杆材质存在缺陷是非正常失效的原因之—
② The material defect of piston rod is one of the causes of abnormal failure

    a. 活塞杆表面存在一定量的杂质及疏松,这些缺陷用锻件标准衡量可能不超标,但对电镀件却是严重的问题(尤其是这些缺陷不是单个的而是集中在一处)。电镀液中含有酸,电镀时,这些酸浸入到杂质及疏松部位中,镀件表面虽然被覆盖上了,但却是搭桥过去的,中间是空的。镀后由于浸入的酸在作怪,镀件很快就会从里向外腐蚀并使其表面成块剥落。有缺陷的表面在电镀过程中使氢析出,形成气泡,造成镀层结合不牢。这就能很好解释为什么有的活塞杆外部涂有油脂,水气一时无法侵入,但不到一个月又锈蚀的原因;也能很好解释为什么用蓝点法测试时无孔隙而褪镀后基体上有蚀坑、裂纹的原因。
a. There are a certain amount of impurities and porosity on the surface of piston rod. These defects may not exceed the standard measured by forging standards, but they are serious problems for electroplated parts (especially these defects are not single but concentrated in one place). There are acids in the electroplating solution. When electroplating, these acids are immersed into impurities and loose parts. Although the surface of the plating part is covered, it is bridged and empty in the middle. After plating, due to the immersion of acid in the trouble, the plating parts will soon corrode from the inside to the outside and make its surface flake. In the process of electroplating, the hydrogen on the surface with defects precipitates and forms bubbles, resulting in the poor bonding of the coating. This can well explain why some piston rods are coated with grease and water vapor can't invade for a while, but they are corroded in less than a month; It can also explain why there are no pores in the blue dot method, but there are corrosion pits and cracks on the substrate after stripping.

b.锻件锻造比过大也有可能造成材质缺陷。锻件锻造比一般要求大于或等于3,而有问题的活塞杆锻造比则达到8以上,因为钢锭存在偏析是不可避免的,所以过大的锻造比就要求钢锭中心线和锻件轴线的一致性较高,以避免钢锭的心部缺陷外露。当我们将有问题的活塞杆返修褪镀又将杆径车小后,发现杆的表面或多或少都存在缺陷,其中有大量的亮线及灰点,也有明显的疏松及裂纹,更有甚者是返修时未褪镀前用砂轮打磨锈蚀处发现基体上就已经存在有裂纹了。而GB/T 112611-90标准明确规定待镀件表面不允许有氧化皮、斑点、凹坑等缺陷。
b. Too large forging ratio may also cause material defects. The forging ratio of forgings is generally required to be greater than or equal to 3, while the forging ratio of piston rod in question is more than 8. Because the segregation of ingot is inevitable, too large forging ratio requires a high consistency between the center line of ingot and the axis of forgings, so as to avoid the exposure of defects in the center of ingot. When we repair the defective piston rod and reduce the diameter of the rod, we find that there are more or less defects on the surface of the rod, including a large number of bright lines and gray spots, obvious looseness and cracks. What's more, before the plating is removed, grinding the rusty part with grinding wheel, we find that there are cracks on the base body. However, GB / T 112611-90 clearly stipulates that the surface of the parts to be plated is not allowed to have defects such as oxide scale, spots and pits.

    c.还有一种材质缺陷也是由加工造成的,有些锻件加工余量过大将表面密实部分加工掉了而露出了钢锭心部的缺陷。锻件校直不够,加工时为了找正只好一边多车一边少车,结果多车那边将密实部分车掉了,而少车那边可能黑皮才刚车掉,这都是不正常的,都会对电镀质量造成不良影响。
c. Another kind of material defect is also caused by processing. Some forgings have too much machining allowance, which will process the dense part of the surface and expose the defect of the ingot core. The forging is not straightened enough. In order to find the correct position, we have to use more cars while using less cars. As a result, the dense part of the car will fall off on the side of more cars, while the black skin may just fall off on the side of less cars. All these are abnormal and will cause adverse effects on the electroplating quality.

    d.锻造专家认为对直径大的40Cr锻件选用520℃的回火温度偏低(专家建议回火温度不低于550℃),不能很好地消除锻件中的应力,并且还应随炉冷却以便进一步地降低锻件表面的残余应力。残余应力是产生微裂纹的根源之一,其不良后果是同镀前不消应镀后不去氢是一样的。
d. Forging experts believe that the tempering temperature of 520 ℃ for large diameter 40Cr forgings is low (experts suggest that the tempering temperature should not be lower than 550 ℃), which can not eliminate the stress in forgings well, and it should be cooled along with the furnace to further reduce the residual stress on the surface of forgings. Residual stress is one of the causes of microcracks, and its adverse effect is the same as that of no stress before plating and no dehydrogenation after plating.

    材质存在的上述缺陷是造成电镀层非正常失效的主要原因,这种先天不足是不能用后天的电镀来弥补的。
The above defects of the material are the main reasons for the abnormal failure of the electroplating layer, which can not be made up by the acquired electroplating.

(4)返修后又很快失效的原因分析
(4) Cause analysis of failure soon after repair

    ①返修后又很快失效的主要原因是褪镀后少了去氢工序。褪镀液中含有较浓的酸,酸造成氢脆,因此,应在褪镀后3h之内进行去氢。褪镀后去氢不只是对40Cr这种对氢敏感的材质,对35钢、45钢及所有褪镀件都必须去氢。褪镀后不去氢可能在下一步电镀过程中就会造成不良影响,这是不能用镀后去氢加以弥补的。
① The main reason for the failure after repair is that there is no dehydrogenation process after deplating. There is a strong acid in the stripping bath, which causes hydrogen embrittlement. Therefore, dehydrogenation should be carried out within 3 hours after stripping. Dehydrogenation is not only necessary for 40Cr, which is sensitive to hydrogen, but also for 35 steel, 45 steel and all parts. The non dehydrogenation after de plating may cause adverse effects in the next electroplating process, which can not be compensated by post plating dehydrogenation.

    ②另一个造成返修后很快失效的原因是返修时增加了电镀时的电流密度。在无实践经验的情况下轻易改变在水工行业用了几十年、电镀过近10万根活塞杆的工艺是有风险的。
② Another cause of failure after repair is the increase of current density during electroplating. Without practical experience, it is risky to easily change the process that has been used in hydraulic industry for decades and electroplated nearly 100000 piston rods.

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