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Vacuum brazing - a high-quality joining process (Part 2) Quality aspects

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Heattreat Column: Vacuum brazing - a high-quality joining process (Part 2) Quality aspects

Quality aspects

Vacuum brazing is a reproducible and controllable process.
The quality assurance can be divided into the following steps:



  • Qualification of the brazing procedure
  • Process registration during production
  • Inspections
For  the qualification of the brazing procedure all important parameters for the braze process must be strictly defined. The objective is to obtain reproducible products, that meet the design requirements. In the brazing procedure, the following parameters shall be qualified:
  • Base  materials
  • Brazement design of  the parts (joint length and geometry)
  • Joint clearance  (braze gap)
  • Precleaning and surface preparation (roughness)
  • Brazing filler material (form and type)
  • Assembly and fixturing 
  • Brazing cycle (temperature, time and vacuum level) 
  • Post braze  heat treatments (if necessary)
  • Nondestructive testing of the brazed joints
  • Mechanical examination of the base material by means of test samples


Brazed heat exchanger stainless steel

During the production process, all process steps must be recorded in measurement reports, furnace records and inspection reports. For all used materials, material certificates should be available. Measure - and production equipment should be calibrated intermittently. Assembly and inspections should be executed by qualified personnel. For nondestructive testing, the minimal requirements must be lay down in a inspection document. It is important that the  brazed joints are accessible for  nondestructive  examinations, so that it can properly inspected.
Non-destructive testing can be divided into the following three methods:

  1. Inspection of deviations in the surface of  the brazed joint: Visual examination, endoscopy and dye penetrant testing
  2. Inspection at braze discontinuities from the brazing joint: radiographic testing, ultrasonic examination. 
  3. Leak testing:
    Pressure test, helium leak detection, leak testing with dye penetrant liquid.



Full automatic ultrasonic equipment

Destructive testing can be executed at samples, which are in the same condition as the brazed material.
 
Destructive tests are usually used to determine the strength of a joint and the base material.
Tension and shear tests have been used to advantage in determining joint strengths for service below and above room temperature. These tests are used more widely for the development than for production quality control. Other destructive tests are; Fatigue tests, Impact test, Torsion tests.

Metallographic examination is frequently used to determine the general quality of brazed joints and to detect such flaws as porosity, poor flow, excessive base metal erosion or improper fit-up.

Part 3 describes applications for different materials. 


Author:Peter Steege 
Brazing Consultancy Peter Steege

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