Common defects in the production process of nickel-based alloy tubes (marketing department of Shanghai HY Industry Co., Ltd)
At present, nickel-based high-temperature alloys are mainly used in aerospace and naval vessels, key hot-end components of industrial gas turbines, important high-temperature structural materials for nuclear reactors, chemical equipment, etc.
Below, Shanghai HY Industry Co.,Ltd takes you to discuss the common defects in the production process of nickel-based alloy tubes.
1、Defects in the smelting and forging process.
Inclusions in nickel-based alloys are mainly composed of brittle corundum Al2O3, aluminates, TiN (O) and semi-plastic silicates and their complex inclusions of Al, Si, Ca, Ti and other elements. These inclusions vary in size, shape and composition, and have different effects on the alloy.
1.2, the inclusions in nickel-based alloys are mainly in the following three forms.
1.2.1, spherical inclusions
Inclusion size from less than a micron to a few microns. These inclusions have little effect on the quality of the alloy when the size is small, but when the size is large, it is not easy to deform because of its brittle, cracking in the spherical and tensile axis orthogonal to the interface, and with the increase in matrix slip, resulting in necking shrinkage leads to crack expansion, the formation of cavities or the generation of microcracks
1.2.2, regular geometry of inclusions
The formation of such inclusions is mainly in the alloy solution before solidification, non-metallic inclusions are solid particles precipitation, with a fairly regular geometry, such as square, rectangular, hexagonal and dendritic.
These inclusions with regular geometry, in the tensile process, the crack first in the tensile direction orthogonal to the inclusions of the sharp corner, and along the inclusions and the matrix interface expansion, resulting in the inclusions and the matrix interface detachment, so the large size of the inclusions with sharp angles on the quality of the alloy is the most harmful
1.2.3, irregular geometry of inclusions
Such inclusions due to the inclusions between the mechanical and physical properties of different, phase boundary bonding force is weak, easy to crack from the phase boundary under the action of tensile stress
2、rolling forming defects
Key point: the choice of glass powder for lubrication
Nickel-based alloys require high matching of softening and melting temperatures of glass powders due to their difficult deformation and poor plasticity. The use of low melting point glass powder will lead to orange peel on the inner wall of the barren tube.
3、Defects in the heat treatment process
Heat treatment is achieved by means of heating, insulation and cooling, if these three means are not well grasped will be the following common problems.
Take N08810 as an example
Overheating of N08810 organization in the residual austenite increased, the dimensional stability is reduced. Due to quenching organization overheating, N08810 steel crystals coarse, will lead to a decline in the toughness of the parts, impact resistance is reduced, the bearing life is also reduced. Overheating is serious and can even cause quenching cracks.
Quenching temperature is low or poor cooling will produce more than the standard taustenite organization in the microstructure, called underheating organization, it makes N08810 hardness decline, wear resistance is sharply reduced, affecting the life of N08810 material.
3.3. Quenching cracking
Causes of this cracking are: due to quenching heating temperature is too high or cooling is too rapid, thermal stress and metal mass volume changes in the tissue stress is greater than the N08810 steel fracture strength; the original defects on the work surface (such as surface micro cracks or scratches) or N08810 steel internal defects (such as slag, serious non-metallic inclusions, white spots, shrinkage residues, etc.) in the quenching of the formation of stress Concentration; serious surface decarburization and carbide segregation; parts after quenching and tempering is insufficient or not timely tempering; the front process caused by excessive cold stress, forging folding, deep turning tool marks, sharp edges of the oil groove, etc.. In short, the cause of quenching cracks may be one or more of the above factors, the presence of internal stress is the main reason for the formation of quenching cracks. Quenching crack organization is characterized by the absence of decarburization on both sides of the crack, clearly distinguishable from forging cracks and material cracks.
3.4. Heat treatment deformation
N08810 in heat treatment, there is thermal stress and tissue stress, this internal stress can be superimposed or partially offset, is complex and variable, because it can change with the heating temperature, heating rate, cooling mode, cooling rate, the shape and size of the part, so N08810 heat treatment deformation is inevitable.
3.5. Surface decarburization
N08810 in the heat treatment process, if it is heated in oxidizing medium, the surface will occur oxidation to reduce the mass fraction of carbon on the surface of the part, resulting in surface decarburization. The depth of the surface decarburization layer exceeds *** after processing the amount of stay will make the parts scrap. n08810 surface decarburization layer depth determination in the metallographic inspection available metallographic method and micro hardness method. To surface layer microhardness distribution curve measurement method prevails, can do arbitration judgment.
3.6. Soft spot
Due to inadequate heating, poor cooling, quenching operation improperly caused by the N08810 surface local hardness is not enough phenomenon called quenching soft spot. It is like surface decarburization can cause a serious decline in the surface N08810 wear resistance and fatigue strength.
4, the defects caused by the pickling process.
For example, over-acid phenomenon: acid pickling N00825 tube with mixed acid (nitric acid + hydrofluoric acid) found that the inner layer is not found over-acid phenomenon, cross-section and outer wall over-acid phenomenon
5、The effect of PT flaw detection on the quality of high nickel-based alloy tubes
Key point: choosing the right penetrant is important
Studies have shown that the element S in the penetrant will produce stress corrosion cracking on nickel-based alloys; many standards specify the content of S in the penetrant as follows: ASME Volume 5 states that for nickel-based alloys, the residual sulfur in the penetrant shall not exceed 1% by weight.