I. Gloss Levels After Polishing Stainless Steel
Through visual inspection, the surface gloss of a polished part is divided into 5 levels:
Level 1: The surface has a dull, white oxide film.
Level 2: Slightly shiny, cannot clearly see the outline.
Level 3: Good brightness, can distinguish the contour.
Level 4: The surface is shiny, you can see the outline clearly (equivalent to the surface quality of electrochemical polishing).
Level 5: Mirror-like shine.
4. Impact of different hardness on the polishing process
An increase in hardness makes grinding more difficult but reduces roughness after polishing. Due to the increase in hardness, the time required for polishing to reach a lower roughness increases correspondingly.
Simultaneously, an increase in hardness reduces the likelihood of overpolishing.
V. Impact of part surface condition on the polishing process
The surface layer of steel may be damaged due to heat, internal stress or other factors during the cutting and machining process, and inappropriate cutting parameters may affect the polishing effect.
The surface after electrical discharge machining (EDM) is more difficult to grind than the surface after ordinary machining or heat treatment, therefore, precise EDM cutting must be done before the end of EDM, otherwise a thin layer hardened will form on the surface.
If the choice of EDM precision cutting is incorrect, the depth of the heat-affected layer can reach up to 0.4 mm. The hardness of the hardened thin layer is greater than the hardness of the base and must be removed.
Therefore, it is better to add a coarse grinding process to completely remove the damaged surface layer, create a uniformly rough metal surface, and provide a good base for polishing.
SAW. Difference Between Shot Peening and Sand Blasting
Shot peening involves launching small balls of steel or iron at high speed to hit the surface of the part, which can remove the oxide layer from the surface of the part.
Simultaneously, the high-speed impact of steel or iron balls causes distortion of the network on the surface of the part, increasing the surface hardness. This is a method of cleaning the surface of the part and is often used to clean castings or strengthen the surface of the part.
Shot peening is generally used for regular shapes where multiple nozzles work together in all directions to achieve high efficiency and less pollution.
In the shipbuilding and repair industry, shot peening and sandblasting are commonly used.
However, both shot peening and sandblasting use compressed air. Of course, shot peening is not necessarily done with a high-speed rotating impeller.
In shipbuilding and repair, shot peening (using small steel balls) is mainly used for pretreatment of steel sheets (removing rust before painting); Sand blasting (using mineral sand in shipbuilding and repair) is mainly used on ships or shaped sections to remove old paint and rust from steel plate for repainting.
The main purpose of shot peening and sandblasting in shipbuilding and repair is to increase the adhesion of the paint to the steel plate.
In fact, not only shot peening is used to clean castings. For large parts, sand cleaning in the drum is usually done first, where the casting is placed in a drum for rolling after the rods are cut. The pieces collide with each other inside the drum, removing most of the sand from the surface before shot peening or sand peening.
The size of the shot peening sphere is 1.5 mm.
Studies have shown that, in terms of damage, metallic materials are much easier to break when there is tensile stress on the surface than when there is compressive stress. When the surface is under compressive stress, the fatigue life of the material increases significantly.
Therefore, for parts such as shafts that are prone to fatigue fracture, sand peening is often used to create surface compressive stress and extend product life.
Furthermore, metallic materials are very sensitive to tension, which is why the tensile strength of materials is much lower than the compressive strength. This is also why tensile strength (yield, tensile strength) is generally used to represent the performance of metallic materials.
The working surface of the steel sheets used in the cars we drive every day is reinforced by sandblasting, which can significantly improve the fatigue resistance of the material.
Blasting is carried out using a motor to drive the rotation of the impeller, and through the action of centrifugal force, pellets with diameters of 0.2 to 3.0 (such as molten shot, cut shot, stainless steel shot, etc.) are shot out on the surface of the workpiece.
This creates a certain roughness on the surface of the piece, making it more attractive. It also changes the tensile welding stress of the part to compressive stress, thus improving the service life of the part.
It is widely used in most areas of machinery, including shipbuilding, automobile parts, aircraft parts, gun and tank surfaces, bridges, steel structures, glass, steel plates, pipes and so on.
Sandblasting (shot) uses compressed air as force to spray sand with diameters between 40 and 120, or pellets around 0.1 to 2.0 on the surface of the part, obtaining the same effect.
The size of the pellets will determine the effect of the treatment. It is important to note that sandblasting can also have a strengthening effect.
Currently, national equipment has entered into a misunderstanding, believing that only blasting can achieve the goal of strengthening.
However, American and Japanese companies use blasting and sandblasting for reinforcement!
Both have their advantages. For example, for a workpiece such as a gear, the shot blast angle cannot be changed and only the initial speed can be changed by frequency conversion.
It has a high volume and speed of treatment, while sandblasting is exactly the opposite. The effect of blasting is not as good as sandblasting.
Sandblasting is a method that uses compressed air to blow quartz sand at high speed to clean the surface of parts. This method, also known as sandblasting in factories, not only removes rust, but can also remove oil as a side effect, which is very useful for painting.
It is commonly used for removing rust from parts surfaces; for decorating the surfaces of parts (small wet sandblasting machines sold on the market are used for this purpose, generally using corundum as the grain and water as the medium); In steel structures, the use of high-strength bolts for connection is an advanced method.
Because high-strength connections rely on friction between the joint surfaces to transmit force, the quality requirements for the joint surface are very high. At this time, sandblasting must be used to treat the joint surface.
Sandblasting is used for complex shapes that are easy to remove rust manually, but it is not highly efficient, the site environment is not good, and the rust removal is uneven.
General sandblasting machines have various specifications of sandblasting guns. As long as the case is not particularly small, the gun can be placed in it to clean it.
The pressure vessel support product – the head uses sandblasting to remove oxide scale on the surface of the workpiece. The diameter of quartz sand is 1.5mm to 3.5mm.
There is a type of processing that uses water as a carrier to transport the diamond sand to process the parts, which is a type of sandblasting.
Both blasting and sandblasting can clean and remove dirt from workpieces, preparing them for the next step. This is to ensure the roughness requirements of the next process, and some are for surface consistency.
Sandblasting has a strengthening effect on workpieces, which is not obvious from sandblasting.
Generally, sandblasting uses small steel balls and sandblasting uses quartz sand. They are divided according to different requirements.
Precision casting uses sandblasting and shot blasting almost every day.
VII. Supplement
1. Both blasting and sand blasting are surface treatments, but not only castings are subjected to blasting.
2. The main function of sand blasting is the removal and deoxidation of surface rust, for example, parts after heat treatment, while sand blasting has multiple functions: not only removing rust and deoxidizing the surface, but also improving the roughness of the surface, removing machining burrs from parts, eliminating internal stress in parts, reducing deformation of parts after heat treatment, increasing surface wear resistance and compression capacity of parts, etc.
3. There are many processes for shot blasting, such as castings, forgings, surface of parts after machining, surface of parts after heat treatment, etc.
4. Sand blasting is mainly operated manually, while sand blasting is often automated or semi-automatic.
5. The steel and iron shot used for blasting are not really shot in the true sense, accurately speaking, they are small steel wires or rods, which only look like shot after being used for a period of time.
The so-called sandblasting sand is nothing more than river sand, identical to construction sand, except that the sandblasting sand is screened, contains less mud and has a specific particle size.
Of course some industries are different, such as the shipbuilding industry, where real steel shot is used for sandblasting and metal ore sand (not river sand – quartz sand) is used for sandblasting.
Additional supplement (some repetitive, others contradictory):
1. Shots versus sand: Shots are generally spherical particles without edges and corners, like cut wire shots; Sand refers to grains with edges and corners, such as brown corundum, white corundum, river sand, etc.
2. Spraying versus throwing: Spraying uses compressed air as force to spray sand or shot-blasted materials onto the surface of the material to achieve cleanliness and a certain level of roughness; Launching uses the centrifugal force generated during high-speed rotation to impact the surface of the material and achieve cleanliness and a certain level of roughness.
