气体保护电弧焊时，使用的保护气体主要有几种，其各自的特点是什么？
参考答案：
保护气体主要有三种：
1氩气：是惰性气体，易引弧，电弧燃烧稳定、柔和，其密度大，易形成良好的保护罩，又因为氩气的原子质量大，所以还具有良好的阴极清理作用。
2氦气：是惰性气体，较氩气比不易引弧，电弧燃烧的稳定性、柔和性也不如氩气，但氩弧的电压高于氩弧，所以氦弧的温度高，发热大且集中，同样条件下焊接速度比氩弧焊高30%～40%，且可获得较大熔深和窄焊道，热影响区也显著减小。
3二氧化碳气体：具有氧化性，来源广、价格低，但要注意其纯度应满足焊接要求。
| 短文改错 此题要求改正所给短文中的错误.对标有题号的每一行作出判断： 如有错误(每行只有一个错误), 则按下列情况改正： 此行多一个词:把多余的词用斜线(\)划掉, 在该行下边横线上写出该词, 并也用斜线划掉。 此行缺一个词:在缺词处加一个漏字符号(∧), 在该行下边横线上写出该加的词。 此行错一个词:在错的词下划一横线, 在该行下边横线上写出改正后的词。 注意:原行没有错的不要改。 | ||
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“Never give up!” It is my law(法则) of life. It has brought me success. I learned the law from my father's famous story. My father was in a poor village in the north of Jiangsu，in China. When he was a young boy，he went to school in the morning，then in the fields till sunset. And then he did his homework during midnight. Life was hard，because they had no ！ At the age of 14，my father heard of the United States of America. It was the land of gold，the land where people can become rich. “ don't I go to America？” he thought to himself，full of hope. So，my father came to America. “I had thought it was easy to collect money in America，” he told me. “But when I arrived there，I realized it was not true. They did not like to hire(雇佣) me because I spoke English. Later，I worked in a small restaurant，cleaning up tables， dishes and sweeping the floor. Life was for the first few years. I worked from 10 a. m. to 11 p. m. I wanted to go to school to learn English，but it was impossible. I couldn't the schooling(学费). ” My father working hard，and reached his goal. “Alan，” he often says to me. “If you want something，you have to work for it and never give up. Things do not come really in life. ” That is what I learned from my father.
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