The application that Ultrasonic flaw detector on weld

 The normal methods of nondestructive detection have  macroscopic inspection that directly keep eyes on  and ray photographic flaw detection、ultrasonic flaw detection/magnetic power 。Penetration flaw detection、eddy current flaw detection。Naked eye macroscopic inspection can without using any equipment and device，but eyes can not penetrate workpiece to inspect workpiece inside flaw   , but ray photographic and other methods can detect by every kind of instrument or device,not only inspect workpiece inside flaw that eyes can not see but also greatly improve detecting accuracy and reliability。As for using what methods to do nondestructive detection,this needs to according to workpiece condition and detecting purpose to make sure。 Ultrasonic wave frequency exceeds human hearing,frequency that more than 20khz acoustic wave is called ultrasonic ,the ultrasonic that used for flaw detection ,frequency is 4-25MHZ,,but 1-5MHZ is mostly used。 Taking advantage of sound   to detect object is good or not,this method has been adopted  by people。Such as ,using hand to beat watermelon to hear whether it is ripe，doctors knock patients’ chest,detecting viscera is normal or not ,using hand to knock China bowl to see whether it is bad，but these detecting methods that according to human hearing to judge is more accurate and objective than acoustic method,and easier to make quantitative e­xpression。Due to ultrasonic flaw detection distance is long,device volume is small,light weight,convenient to be taken to on-site inspection,detecting speed is fast ,but in procession it just consumes coupling agent and has abrasion on probe,the whole inspection costs is low,currently   construction market is mainly adopt this method to detect。At first,knowing about requisition that drawing on weld quality，currently,acceptance standard of steel structure is according to GB50205-95 《steel structure engineering construction and acceptance regulation》to execute。Standard stipulates that drawing require welding joint weld quality level is one level,assessing level is II,regulation stipulates to do ultrasonic flaw detection,for drawing requires welding joint weld quality level is second level,assessing level is III,regulation stipulates to do20% ultrasonic flaw detection，for drawing requires welding joint weld quality is third level,not to do ultrasonic inside flaw inspection。

It is worth noting that ultrasonic flaw detection is applied  to  full penetration weld,its flaw detection proportion is figuring according to each welding joint length percentage ,and not lower than 200mm,for partial flaw detection welding joint ,if discovering unallowed flaw ,it should improve flaw detecting length at extended parts of both sides of flaw,improving length should not lower than its welding joint 10%,and not lower than 200mm,when still has unallowed flaw,it should do flaw detection inspection on welding joint，secondly,clearly know good time of detecting，carbon structure steel should cooling to environmental temperature ,low alloy structure steel can do welding joint flaw detection inspection after finishing welding for 24 hours,in addition,we should know waiting for detecting workpiece parent metal thickness 、connector type and groove type,until now ,in my practical job, most of welding joint that needed to be detected are connector type that middle plate to welding joint，so I will aim at operation on welding joint flaw detection to make summary。General parent metal thickness is between 8-16mm, groove type has I  type,single V type,X type,only after figuring out these things can we begin to do preparation before flaw detection。

Before each flaw detection operation,needing to use standard test block to calibrate comprehensive properties of instrument，calibrate panel curve to make sure accuracy of flaw detection result  。

Concrete methods as follows:

1 、detecting surface repair: eliminating weld surface glass splinter 、oxide skin、pit and rusting,surface perfection normally lower than ▽4。Both sides detecting surface of Welding joint repair width normally more than or equal to 2KT+50mm (K: probe K value,T: thickness of workpiece),normally according to weldment parent metal to choose probe that K value is 2.5,such as waiting for detecting workpiece parent metal thickness is 10mm,then repair100mm at both sides of welding joint。

2、choice of coupling agent should consider viscosity 、mobility、adhesive force、have no corrosion on workpiece surface、easy to clean,and economical，according to above factors to choose paste as coupling agent 

3、Due to parent metal thickness is thinner so this detectin g direction adopts single face double sides。

4、Due to thickness of plate is lower than 20mm,so adopting ,horizontal positioning to adjust scanning speed of instrument

5、 in the process of operating,adopting thick flaw detection and precision flaw detection 。For probably know about flaw is exist or not and distributed condition、quantify、positioning is precision flaw detection，using sawtooth shape to scan、lateral scan、before and after scan、angle scan，circumferential scan so as to be convenient to find every kind of flaw and judge flaw property。

6、recording detecting result,if inside flaw is found ,then assess and analyze。Weld  classifies inside flaw should according to national standard GB11345-89《steel welding joint manual ultrasonic flaw detection method and detecting result classification》To decide whether it is qualified。 If excessive flaw is found,giving  rectification notice to workshop,let them abarbeitung to  re-inspect until qualified

Welding joint testing method:

Welding joint testing methods:1、appearance observing2. Compactness test and water pressure test3. Welding joint ray photographic4. Ultrasonic flaw detection 5. Magnetic flaw detection 6. Penetration flaw detection ,in regard to return to repair regulation:specific condition should be particularly treated,anyway try to reduce times of returning to repair, weld quality of steel structure is very important,nondestructive is an important way to make sure steel structure weld quality,Normal flaws in general welding joint :air hole、slag inclusion、lack of penetration and crack,until now there is no mature method to judge accurately on flaw property，just according to the shape of flaw wave on the fluorescent screen and reflectivity height change combined with flaw position and weld technology to do comprehensive assessment。

For assessment of inside flaw and prevention and reason of flaw generating mainly have as follows methods:

1.Air hole:

Single air hole echo height is low ,waveform is monolete suture ,rather stable。From all direction to detect,reflectivity is almost the same， but once moved ,the probe will be disappeared，density air hole will produce a bunch reflectivity ,wave height is different with different size of air hole， when probe rotates as fixed point，it will appear this ups and that downs 。 The reason of generating this kind of flaw is mainly welding material didn’t drying according to regulation temperature，welding rod is metamorphic and fall off、core wire corrosion、solder wire didn’t cleaned，manual weld ,electricity is strong，electric arc is large，   submerged-arc welding  ,voltage is strong or net work voltage fluctuation is too big，gas shielded welding, protective gas purity is low。If there is air hole in welding joint,not only destroy welding joint metal compactness,But also reduce welding joint effective sectional area，reduce mechanical properties，especially this air hole,reduce curve and impact thoughness,prevent these flaw has methods:not use welding rod with coated cracking , flaking 、metamorphic and core wire corrosion， rusty welding stick can be used only do rust removal， welding material should be dried according to regulation temperature，groove and its two sides cleaned up ，and choose suitable welding current、arc voltage and welding speed。

2. Slag inclusion

Dot slag echo signal is similar to spot shape air hole，strip slag inclusion echo signal presents as hackly ,and amplitude is not high，waveform presents as arborization,by the side of main peak has small peak,probe translation ,amplitude has changes，reflectivity amplitude is different when detect from all directions。

The reason of generating this kind of flaw :welding electricity is too weak，speed is too fast, slag has no time to float，welding edge and every level of welding joint didn’t cleaned up ,its metal and welding material chemical ingredient is not appropriate，including sulfur、phosphor。Preventing methods:correctly choose welding current，groove angle of weldment should not too small，before welding ,the groove must be cleaned up，multiple layers weld ,must clean welding slag layer by layer，and properly choose welding speed 。

3. Pack of weld penetration 

High reflectivity ,amplitude is high as well,waveform is stable，by the sides of welding joint can get approximately the same reflectivity amplitude when flaw detection。This flaw is not only reduce welding connector mechanical properties，but also form focal point of stress,after bearing,it always cause crack that is a dangerous defect。

General reason:beveled edges has little interval，welding current is too low,speed is too fast，beveled edges angle is small, arc blow

Preventing methods:properly choose beveled edges type、assemble interval and adopt correct welding technology。

4. Incomplete fusion  

When probe translation，waveform is stable，detect from both sides,reflectivity amplitude is different，sometimes just can be detected from one side。

Reason of generating :beveled edges is not clean,welding speed is too fast,electricity is too low or too big，welding rod angle is not correct，arc blow

Preventing methods:correctly choose beveled edges and electricity ,beveled edges is cleaned, correctly operate and prevent weld misalignment。

5.Crack:  

Echo height is bigger，amplitude is wide，appear many peaks，probe translation ,reflectivity continuously emerge fluctuant amplitude，probe rotates,wave crest has fluctuant phenomenon，crack is the most dangerous flaw, it not only reduces strength of welding connector ,but also presents sharp gap due to crack，after weldment bearing,lead to stress concentration，and become the origin of structure  tear，crack divided into hot crack，cold crack and reheating crack。

The reason of generating hot crack :when welding, molten pool has quick cooling speed,generating segregation，welding joint has uneven heating and produces  tensile stress。

Preventing methods: restrain segregation factors and content of harmful impurity in parent metal and welding material ，mainly restrain sulphur content ，improve manganese content；improve welding rod or basicity of soldering flux so as to reduce impurity content。Improving segregation level,improve welding structure formation，adopting reasonable welding order to improve contraction of degrees of freedom。

The reasons of generating Cold crack :being welded material   has big hardenability ,it is easy to crack when it is in welding tension by human in process of cooling ,during welding,cooling speed is too fast for hydrogen to get out and remained in welding joint，hydrogen atoms into hydrogen molecules,enter into small gap in metal as gas condition，and generate much pressure，make partial metal generates much pressure and produces  cold crack。

Preventive measure : weld preheating,slow cooling after welding Make heat affected zone austenite exploded can be done in  enough temperature interval,avoid quenched structure from producing，simultaneously,reduce the function of welding stress,   after welding instantly do low temperature annealing, hydrogen   relief treatment,eliminate stress that produced at the time of welding,and make hydrogen spread to the outside world,choose  low hydrogen type welding rod and basic flux or austenite  stainless steel welding rod,the welding material according to regulation to dry,and strictly clear groove,strengthen protection of welding and clean on the surface of being welded,avoid hydrogen from entering ,choose reasonable welding regulation,adopt reasonable order,so as to improve welding condition