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purpose of preheating before welding [复制链接]

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只看楼主 倒序阅读 使用道具 楼主  发表于: 2016-09-10
I have a question to need your help
Q345R that is  similar to S355J2 do not need preheat before welding when the base metal thickness less than 30mm according to PQR and our experience,recently my colleagues,a Frenchman,from Germany headquarters ,tell me that we should preheat befor welding,he said preheating is better,he also tell tack welding is no problem without preheat,it confuse me. In my opinion,preheat can prevent weld from
delayed crack due to quick cooling,so we preheat when we weld more thick plate,I think that tack welding have a much quickly cooling,however he told me you can no preheat before tack welding.
Can you help to explain why he so say?
Thanks in advance!

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只看该作者 沙发  发表于: 2016-09-11
我认为就只是一个老外对一个位于灰色地带的问题的个人的看法和认知而已,没有必要做过多的解读。
就像你上面贴子里说的:preheating is better。Q345,SJ355这样的材料,焊接性并没有坏到不预热就一定会裂,也没有好到你不预热也一定没问题的程度。标准上虽然有推荐做法,达到多厚需要预热到多少度,但根本上,还是决定于工程师对风险的评估。通行做法是:预热肯定是不会错的。
你认为点焊冷得快,所以要预热防止裂纹,是对的。我们通常都是这样的看法。老外说没有必要预热,如果没有直接违反规范和作业程序,而且结果也证明,这样做的风险也是可控的,那他的说法也是对的。
在遵守规范和要求前提下,一切基于对风险的评估,而采取有效且经济的办法。
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hawk_hawk 金钱 +15 热心应助! 2016-09-12

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只看该作者 板凳  发表于: 2016-09-11

please read BS EN 1101-1 & 2, BS EN ISO 13916, you will now the preheat requirement.

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只看该作者 地板  发表于: 2016-09-11
Maybe you tack weld get removed when final welding.

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只看该作者 4楼 发表于: 2016-09-11
回 pyfhbes00 的帖子
now I want to find a reasonable explaination that can persuade him accept our welding procedure,because according to his suggestion,productivity is too low, even though quality maybe is better,he will ask me to preheat every day if I can not persuade him,he will stay here untill  December.

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只看该作者 5楼 发表于: 2016-09-11
回 playboy1898 的帖子
it does not matter with if tack weld get removed

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只看该作者 6楼 发表于: 2016-09-11
回 hejing0412 的帖子
the three standard describe:
1.EN1011-1 require that tack welding and welding have same preheat requirement
2.ISO13916 give operational requirement of preheat ,including preheat area and so on
3.EN1011-2 give some suggestion and method which decide preheat  temperature

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只看该作者 7楼 发表于: 2016-09-11
回 525708290 的帖子
If your job follow U.S CODE, maybe AWS D1.1 charpter 3 preqaulify preheat tempature also can be followed.

If you want save time and cost to avoid preheat, you have to qualify that:

PQR withourt preheat and thickness equal to the product and the stress also same, then, your hardness test value lower than the requirment at BS EN ISO 15614, your owner should be accept.

So, hardness test on the product also help, normally HAZ hardness value will be highest area.

Such as AH 36 to FH 36, 350 HV10 was required normally, for offshore product hardness lower than 325 HV10 much better.

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只看该作者 8楼 发表于: 2016-09-11
回 525708290 的帖子
You should follow the standard system your Spec. mensioned.

BS , EN, ISO, ASME, JIS, AS , AWS, something like this.

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只看该作者 9楼 发表于: 2016-09-11
Re: Preheating
If you need to persuade him to accept you welding procedure, you need to show why you do not do the preheating.
1. cauculate the CET.
2.thickness of the material
3. HD
4. heat input KJ
Then  calculated according to the following formula:  C.8 in EN 1011-2 Page 36
Tp=697*CET+160*tanh (d/35)+ 62 *HD (0,35)+(53*CET - 32) *Q-328  
This relationship is valid for structural steels with a yield strength up to 1 000 N/mm2 and
CET = 0,2 % to 0,5 %
d = 10 mm to 90 mm
HD = 1 ml/100g to 20 ml/100g
Q = 0,5 kJ/mm to 4,0 kJ/mm
Preheating Temperature Calculate.zip (3 K) 下载次数:12

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只看该作者 10楼 发表于: 2016-09-11
回 playboy1898 的帖子
Code and standard and Specification will persuade ower according to contract, otherwise, the owner may do not follow.

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只看该作者 11楼 发表于: 2016-09-11
He is WRONG.  Preheat is even more critical prior to tack welding due to the even quicker heat up of a cool part and much quicker cool down of that cool part.  The micro structure effects will be more extreme than when actually running a weld pass that has an opportunity to heat up the area itself even though the first part has not had that advantage.  

Pre-heat does so much to assist in sound weld deposits.  It is easy to find this in any book on basic welding.  The AWS Handbooks on Welding series, I think the 1st on is the Science and Technology one with much good info.  Then, in Linnert's 'Welding Metallurgy' you can find a lot of information.  

Yes, the thicker the material the more important pre-heat is and the higher it needs to be, both for tacking and for welding.  One of the main purposes is to assist with how that material cools coming down and going back up through the phase temperature area.  All of it has to do with how the HAZ will be effected as well as hydrogen being drawn into the weld pool causing Hydrogen, underbead, cracking during cooling.  

When and if pre-heat is needed will depend upon ambient air temperature (humidity can also come into play but isn't included in the codes for the most part) as that will determine if your parts are already at the required heat for the thickness encountered.  But, there are times it is advantageous to add preheat even when the parts are at the code required heat.  You can't go wrong adding a little heat but you can really go wrong by not using it when needed.  

Hope this helps.  In a bit of a hurry so may not have really explained it well.  Mainly trying to clear up any confusion.
上面的是一个美国人的回答

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只看该作者 12楼 发表于: 2016-09-12
请输入描述 S355MC.pdf (112 K) 下载次数:3

昨天晚上临时搜到老外钢厂给的产品信息
第二页提出,这个系列钢材预热除非在工件温度低于5度时,否则没必要






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只看该作者 13楼 发表于: 2016-09-12
有条件,也可预热。

[ 此帖被狂超临界在2016-09-14 09:21重新编辑 ]

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只看该作者 14楼 发表于: 2016-09-12
很高兴看到这么多英文讨论。预热的问题一直是老生常谈,楼主大有秀才遇到兵的苦衷,大家的讨论都极为正点,剩下的就是对老外的公关了。建议首先内部要统一意见,上中下策预设好后,就是如何“蔺相如”式地表达了。
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