The quality of welding is determined by how durable the seam is. At the same time, the connection control should be non-destructive, which forces the masters to turn to special verification methods after the operation is completed. In some cases, an external inspection without devices is allowed, but in the case of critical structures, visual control should not be limited. It can be supplemented by color flaw detection of welds - one of the most effective, convenient and reliable methods for the analysis of such joints.
Features of the verification method
This method of checking welds refers to capillary, but focuses exclusively on external troubleshooting. That is, with its help it is impossible to check the internal structure of the joint zone, in contrast, for example, from ultrasonic flaw detection. Typically, these are susceptible to cracking metals. In addition, color control allows you to efficiently identify surface discontinuities.
As for the principle of verification, it is based on the sensitivity of metals to interaction with flaw detection non-ferrous materials. Contrary to popular belief, the analysis does not use the characteristics of the surface of the workpiece as controlled information, but rather the changes provoked by irritating the structure with chemicals. Based on the processes of chemical exposure, the method of color defectoscopy is based, which allows you to process information about changes and, at the output, provide data on the identified defects of the metal.
The method itself is rarely used alone. Usually it is used in the general verification complex - along with ultrasound. First comes a color analysis, and then an ultrasonic flaw detector.
Applicable Materials
As already mentioned, flaw detection is not complete without the use of special chemicals. The standard set for color flaw detection includes three types of components: an indicator penetrant, a cleaning agent, and a developer. That is, the color sensitivity control will be activated due to the three-stage processing of the working area. At each stage, special materials are used.
Penetrants are, in fact, coloring substances that can be presented in different forms. Powder products are also used, but for their use, additional operations of thermal exposure or mixing with liquids must be introduced. Penetrants suitable for a particular metal for color flaw detection are selected based on several operational properties. It takes into account, in particular, refractoriness, environmental friendliness and toxicity, eliminating the need for subsequent cleaning, etc.
Requirements for the place of control
A defectoscopy operation can only be carried out in a specially prepared room. It is important to take into account the requirements for the communication equipment of the place, to the characteristics of the air, technical support, etc. The room must be ventilated, not have sources of open flame or intense heating. Lighting should be general and local, luminaires close to the operation site should be protected from thermal effects.
Initially, care should also be taken to create an optimal microclimate - only in favorable conditions can a color defectoscopy be carried out qualitatively. GOST 18442-80, in particular, indicates that the air should be dry and warm - if necessary, to achieve these characteristics, you can use an infrared heater that will allow the developer to dry at 5 ° C. The technical equipment will depend on the methods of applying the chemical components. In particular, spray guns, spray cans, brushes, etc. can be used to apply the same penetrant.
Suture preparation
The joint section is also being prepared for a flaw detection operation. If there are pronounced roughness or contamination on the surface, then you can use fine-grained material and make easy processing. Next, degreasing is performed. For this task, you can use one of the components of the flaw detection kit, but it is important that it provides the effect of degreasing, and even better - increases the sensitivity of the control. It is not recommended to use kerosene for such purposes, but gasoline and acetone can be used. If there is no ventilation in the room, degreasing is carried out only with aqueous solutions based on powdered synthetic means of low concentration. If color flaw detection of a small part is planned, then full immersion in a degreasing compound can also be used. In other cases, either a spraying technique is used on the target area, or application with rags soaked in a solution.
Penetrant application
First of all, indicator penetrant is applied. As in the case of degreasing, this procedure can be performed by spraying, dipping or brushing - it depends on the characteristics of the workpiece. Moreover, the application should be carried out in several layers (4-6) and so that each previous layer of the composition does not have time to dry. Also, so that the color flaw detection of welds shows an optimally accurate result, each subsequent layer should cover a large area relative to the previous one. This is due to the fact that each new layer will dissolve the contour of the previous spot, avoiding sharp transitions and influxes, which can be perceived as false cracks. If the operation is performed in conditions of negative temperature, then the penetrant itself should have a state of the order of 15 ° C.
Penetrant removal
Immediately after wearing, the indicator film should be removed using a clean cloth or cloth that does not have a rough pile. Previously, the material must be moistened in ethanol and the surface must be cleaned until the coating is completely eliminated. Problems can arise if work is carried out on the surface of a rough metal - in which case it makes sense to use light abrasives. Next, an oil-kerosene mixture is used. It covers the entire target area, and then is also removed with rags or napkins. Throughout the two steps described, color inspection must maintain a clean and dry surface around the seam. This is important to maintain the accuracy of further monitoring procedures.
Developer Application
As in the cases with indicator layers and cleaning compounds, the developer is stacked in different ways - from the brush to the spray guns. The main thing at this stage is to observe the uniformity and monolithicity of the formed coating. Therefore, it is necessary to carefully check the spraying tools, nozzles and other elements of the devices that affect the quality of delivery of the product to the target location. All this in the future will affect the control of color flaw detection, as well as the quality of subsequent verification operations. After applying the developer, drying is performed. It may not be necessary to wait for polymerization in vivo, but low-power point heaters are allowed to speed up the process.
Inspected Area Inspection
When the developer dries, you can prepare for its inspection by waiting another 30 minutes. Further control by color inspection involves the use of a magnifier that provides a minimum of 5-fold increase. If the layer-by-layer control technique is used, then the check can be carried out already 2 minutes after the processing by the developer.
In the process of evaluation, the operator fixes the characteristics of the spot left - in particular, writes the size of the contours, shape, etc. The main task that color defectoscopy sets is to detect surface flaws that clearly manifest themselves after the procedure is completed. Mechanical damages, and traces of corrosive effects, as well as cracks with excessive influxes can be detected.
Flaw Detection Safety
Requirements for safety measures are caused by threats of fire and explosion during flaw detection. Therefore, it is necessary at least to prepare overalls that include a cotton dressing gown, a hat and rubber gloves, the surface of which must be covered with talc. On the site itself, safety measures must be fully observed. During the procedure, there should be neither open sources of fire, nor sparks. If color capillary flaw detection is carried out regularly in the same room, it is necessary to install appropriate signs on the prohibition of smoking and compliance with fire protection measures. Rags, wipes and flammable products should be stored in a refractory container.
Conclusion
It will be wrong to consider this method of assessing the quality of the weld as optimal, choosing it from the general spectrum of other methods. This is a specially directed survey method that is focused specifically on the analysis of the surface of the workpiece. That is, color inspection is not a comprehensive control measure, but with relatively high efficiency provides information about external flaws. If this method is supplemented by other methods for studying the internal structure of the weld, then complete information about the structure of the metal can be obtained. Next, a decision is already made on a possible correction of the identified defects. In some cases, secondary welding is performed - this will depend on the number and significance of defects in terms of the effect on the strength of the workpiece structure. Sometimes faults of this type are also used to analyze the structure of old joints and joints in operating structures.