What Are Our Concerns About Bearings（54）

Q1:What is the difference between Q235 material and 20# forgings grade II standard material?

A:The main difference is the content of Mn. The content of Mn of Q235 material is below 0.6, and that of 20# material is 0.6-1.05  

Q2:If there are tolerances on the part drawing, consider whether the tolerances should be according to the part drawing when designing the forging drawing.  Are the two tolerances consistent?  

A:Drawings marked are tolerance, just choose according to precision is marked or not marked, usually require high precision or the size of the processing requirements are marked tolerance, rather than important size is generally not mark, mark said there was no tolerance requirements, usually indicated in the technical requirements not note tolerances on the drawing standards (gb, standard or enterprise standard),  The forging tolerance is part tolerance, if not, the tolerance is not the same.  

In short, forging size is a national standard, line standard or enterprise standard, forging allowance is also a national standard for reference, the size of the margin depends on product accuracy, dimensional tolerance, material shrinkage, processing technology, the principle is to choose the minimum processing allowance under the premise of ensuring product quality  

Q3:What are the machining allowances and tolerances for forgings?

A:Machining allowance and tolerance GB/T 15826 for steel free forgings on hammer

Machining allowance and tolerance JB/T 9179 for steel free forgings on hydraulic presses

Q4:What are the distinguishing methods of valve castings and forgings?

A:Castings and forgings:  

• 1, the surface structure of the fine density of the forging, the surface of the casting should be thicker, and the machining surface of the forging should be brighter.  
• 2, cast iron section gray color, and forged steel section silver bright.  
• 3. Listen to the sound, the forging is dense, the sound is clear after the blow, and the sound of the casting is dull.  
• 4, with grinder grinding to see the two sparks with different (forgings bright) and so on.  

Q5:What is the difference between machined parts and die forging parts?

• A:1. If the dimensional accuracy and surface roughness meet the requirements of the design or standards or acceptance conditions, the internal quality of die forging is generally better than that of machining, which also depends on the special design of the die.  
• 2. Batch and cost: if the batch is large enough, the general die forging parts have lower cost, higher efficiency and material saving than the machined parts.  Small batch or several pieces, with free forging billet, low machining cost.  
• 3. Special precision parts: machining can reach 0.01MM level, and die forging is 0.10 level (as far as I know, it is very high)  

Q6:Is forging flaw detection grade ONE or grade two good?

A:Of course, is the ⅱ class is good!  

China’s general carbon steel forgings are jB4726-2000 standard, grade I only as long as the hardness can meet the standard.  In addition to the hardness of class ⅱ, tensile and impact test requirements, and meet the requirements of the standard.  

Low temperature forgings adopt JB4727-2000 standard.  

Stainless steel forgings adopt JB4728-2000 standard  

And inspection, are generally ⅲ, ⅳ level only need to do.  Grade ⅰ and ⅱ forgings do not need to be tested.  

Q7:How is forgings ultrasonic inspection carried out?

A:Forgings should generally carry out longitudinal wave inspection, simple forgings should also carry out shear wave inspection, but the scanning site and acceptance standards should be agreed by the supply and demand.  

Scanning method:  

• 1. In principle, forgings should be tested in two perpendicular directions to detect the total volume of forgings as far as possible. The main detection direction is shown in Figure 2, and forgings of other shapes can also be performed by reference.  
• 2. Sweep scope: the whole surface of the forging should be continuously and comprehensively swept.  
• 3. Scanning speed: the probe moving speed shall not exceed 150mm/s.  
• 4. Scan cover shall be more than 15% of the diameter of the probe.  
• 5. When the detection thickness of the forging is greater than 400mm, it should be tested from the opposite two ends.  

Test of flaw detection sensitivity:  

• 1.In principle, large flat bottom is used to determine the flaw detection sensitivity by calculation method. For the workpiece limited by geometric shape and the defect in the near field area, test block method can be used.  
• 2.The sensitivity should be corrected with the bottom wave method, and the position of the correction point should be selected as the intact area without defects on the workpiece.
• 3.surface compensation: for the workpiece whose detection surface is curved and the bottom wave method cannot be adopted, reference blocks whose curvature is the same or close to the workpiece (0.7-1.1 times) should be used;  Or use the probe of small diameter wafer, so that the length of the near field area is less than or equal to 1/4 of the workpiece radius, so that there is no need for surface compensation.  
• 4.Flaw detection sensitivity shall not be less than φ 2mm equivalent diameter.  

Determination of defect equivalent:  

• 1. AVG curve and calculation method are used to determine defect equivalent.  
• 2. When calculating defect equivalent, when material attenuation coefficient exceeds 4dB/m, correction should be considered.  
• 3. Determination of material attenuation coefficient