问题 问答题 简答题

铸铁焊补的主要问题是什么?

答案

参考答案:

(1)熔合区易产生白口组织。由于焊接时为局部加热,焊后铸铁件上的焊补区冷却速度远比铸造成形时快得多,因此很容易形成自口组织,其硬度很高,焊后很难进行机械加工。

(2)易产生裂纹。铸铁强度低,塑性差。当焊接应力较大时,就会在焊缝及热影响区内产生裂纹,甚至使焊缝整体断裂。此外,当采用非铸铁组织的焊条或焊丝冷焊铸铁件时,铸铁因碳及硫、磷杂质含量高,基体材料过多熔入焊缝中,易产生裂纹。

(3)易产生气孔。铸铁含碳量高,焊接时易生成CO和CO2气体,铸铁凝固中由液态转变为固态所经过的时间很短,熔池中的气体来不及逸出而形成气孔。

此外,铸铁的流动性好,立焊时熔池金属容易流失,所以一般只应进行平焊。

选择题
写作题

第二节 读写任务(共1小题,满分25分)

阅读下面的短文,然后按照要求写一篇150词左右的英语短文。

Beijing is set to lift its12-year-old ban on fireworks for the traditional Spring Festival holidays, probably starting from next Chinese Lunar New Year in January 2006.

Zhou Jidong, director of the Legislative Affairs Office of the Beijing municipal government(市政府), told a press conference last Thursday that his office has completed the draft(草案) regulations on firework safety supervision.

Under the draft regulations , setting off fireworks will be allowed in the capital city’s eight downtown districts from the Eve of the Lunar New Year to Lantern Festival 15 days later. When the draft regulations were posted on the municipal government’s website for suggestions, local residents gave a total of 969 pieces of advice, with 70 per cent in favour of lifting the ban. Related surveys suggest local people who want to light fireworks during Spring Festival range from 50.4 per cent to 86.3 per cent.

In fact, the voice of opposition and criticism has never ended since the municipal government introduced the ban in 1993, considering pollution, noise, fire worries and personal injuries caused by letting off fireworks.

Opponents accused the government of introducing the ban only for the sake of facilitating its management role while disregarding the will of the people. Worse, they say, the ban threatened to break down the age—old traditions of celebrating Spring Festival among Chinese people.

Partly due to this endless objection, about 106 out of a total 282 Chinese cities that had followed Beijing’s lead in banning fireworks have already dropped the ban.

[写作内容]

1.概括短文的内容要点,该部分的字数大约30词左右;

2.就“should the ban be lifted”这个主题发表你的看法,至少包含以下的内容要点,该部分的字数大约120词左右;

1)解禁的利弊;

2)你的看法如何;

3)你有什么建议?

[写作要求]

你可以使用实例或其他论述方法支持你的论点,也可以参照阅读材料的内容,但不要抄袭阅读材料中的句子。

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