What is a dip coating crack
What is a dip coating crack
It is common sense to dip after pickling. At this time, if the parts with residual stress are pickled, corrosion cracks will occur due to acid corrosion. This kind of crack is called pickling crack or dip coating crack. It is also called stress corrosion cracking. There are two kinds of residual stresses: one is the quenching stress produced by quenching; Another kind of residual stress is the mechanical stress caused by bending.
No matter what kind of residual stress belongs to tensile stress, and cracking is caused by this tensile stress. For example, during bending processing, cracking occurs from the concave side because there is tension on the concave side. Pickling crack is the result of hydrogen action in acid solution and a kind of hydrogen embrittlement. Therefore, before pickling, the galvanized parts must be annealed to eliminate the stress. Structural steel (SS41) parts are often dip galvanized. For example, the parts processed by blanking or bending are dip galvanized after direct acid flow. Even if most parts do not crack, they cannot be used due to reduced strength. At this time, 500 – 600℃ stress relief annealing must be carried out before dip coating. The higher the hardness of the parts, the more sensitive they are to dip coating cracks.
For parts with hardness exceeding HRC50, special attention shall be paid to operation. In order to prevent dip coating cracks, stress relief annealing must be carried out before acid washing. The quenched and tempered parts must be annealed at about 600 ℃. When annealing treatment cannot be used, shot peening hardening method can be used for treatment. Shot peening hardening can produce residual compressive stress on the surface layer, so as to prevent cracks.
Most of the hot-dip cracks appear during or immediately after pickling, and some of them appear in the form of delayed cracking after a long time. The hot-dip cracking of hot-dip parts is accompanied by hot-dip brittleness (see below), which must also be considered.
Seasonal Crack (aging crack) can also be caused when there is residual stress in not only steel, but also red copper parts processed at room temperature. The method for checking the Seasonal Crack of brass is the mercury homogeneous method. The brass parts not annealed at 200 ℃ are immersed in mercuric nitrite aqueous solution for 15 minutes to see if cracking occurs. If there is residual stress, Mercury will form amalgam on the surface of brass parts, and the embrittlement of amalgam will produce cracking. Mercuric nitrite aqueous solution is a solution containing 100 g mercuric nitrite and 13 ml nitric acid in 1 liter of water. Therefore, this test can test the residual stress like acid pickling.
Why do hot dip cracks occur
The dip galvanizing crack occurs when the parts with residual stress are pickled. It belongs to hydrogen embrittlement crack. In the residual stress, this crack is shown only when there is tensile stress. Therefore, parts with high quenching hardness (HRC greater than 50) are prone to galvanizing cracks. Residual stresses are usually phase transformation stress and machining stress.
How to prevent galvanizing crack
During hot-dip galvanizing, stress relief annealing must be carried out for parts before pickling. The temperature of stress relief annealing should preferably be controlled in the range of 450 – 650 ℃. If the part cannot be heated, it is best to use shot peening hardening or shot peening to eliminate stress. Mechanical vibration stress relief is also recommended.
What is a spring dip crack
It is common sense that springs are usually dipped after pickling. At this time, if the spring with residual stress is pickled, corrosion crack will occur, which is called pickling crack or dip coating crack. There are two kinds of residual stresses in electrical corrosion cracking: one is the quenching stress caused by quenching, and the other is the mechanical stress caused by bending processing. No matter which kind of residual stress belongs to tensile stress, and cracking is caused by this tensile stress.
For example, during bending processing, there is tension on the working side, so cracking occurs from the concave side. Pickling crack is the result of hydrogen action in acid solution, which is a kind of hydrogen embrittlement. Therefore, the UN plated spring must be annealed to eliminate stress before pickling.
Structural steel parts are often galvanized. For example, the springs processed by blanking or bending are diffusely plated after acid pickling. Even if most springs do not crack, their strength will be reduced and can not be used. At this time, 500 – 600 stress relief annealing must be carried out before iron dipping. The higher the hardness of the spring, the more sensitive it is to the unplated cracks.
Special attention shall be paid to the operation of springs with hardness exceeding HRC50.
In order to prevent unplated cracks, stress relief annealing must be carried out before pickling. The quenched and tempered spring must be annealed at about 600C. When annealing treatment cannot be used, shot peening hardening method can be used for treatment. Shot peening hardening can generate residual compressive stress on the surface layer to prevent cracks. Most of the dip coating cracks appear during or immediately after pickling, and some of them appear in the form of delayed cracking after a long time, The non plated spring is accompanied by plating brittleness while the dip coating crack occurs, which has to be considered.
Seasonal Crack (aging crack) can also be caused when residual stress exists in not only steel, but also brass spring after normal temperature processing. The method of inspecting brass cracks is the mercury method. The brass spring not annealed in 2001c is soaked in mercuric nitrite aqueous solution for 15 minutes to see if cracking occurs. If there is residual stress, Mercury will record on the surface of brass spring, and Paiqi embrittlement will produce cracking. Mercuric nitrite aqueous solution is quenched spinach containing 100 g mercuric nitrite and 13 ml nitric acid in 1 liter of water. Therefore, this test can test the residual stress like acid pickling.
Source: China Pipeline Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]
