What is Solution Treatment?

What is solution treatment?Solution treatment refers to the heat treatment process in which the alloy is heated to a high-temperature single-phase region and maintained at a constant temperature, so that the excess phase is fully dissolved into the solid solution and then rapidly cooled to obtain a supersaturated solid solution.

Because the operation process is similar to quenching, it is also called “solution quenching”. It is suitable for alloys based on solid solution and whose solubility changes greatly when the temperature changes. First, the alloy is heated to a suitable temperature above the solubility curve and below the solidus line for a certain period of time to dissolve the second phase into the solid solution. Then rapidly cool in water or other media to inhibit the second phase from re-precipitating, and then a supersaturated solid solution at room temperature or a solid solution phase that usually only exists at high temperatures can be obtained. Due to the thermodynamically metastable state, precipitation or other transformations will occur under appropriate temperature or stress conditions. Generally belong to preliminary heat treatment, and its function is to prepare optimum conditions for subsequent heat treatment.

Solution Treatment theory:

Solid solution treatment is to dissolve the carbides in the matrix and γ’ to be equal to obtain a uniform supersaturated solid solution, which facilitates the re-precipitation of fine, uniformly distributed carbides and γ’ and other strengthening phases during aging, and at the same time eliminates due to hot and cold processing. Stress causes the alloy to recrystallize. Secondly, the solution treatment is to obtain a suitable grain size to ensure the high temperature creep resistance of the alloy. The temperature range of solid solution treatment is about 980~1250℃, which is mainly selected according to the phase precipitation and dissolution rules in each alloy and the use requirements to ensure the necessary precipitation conditions and a certain grain size of the main strengthening phase. For alloys used at high temperature for a long time, better high temperature durability and creep properties are required, and a higher solution temperature should be selected to obtain a larger grain size; for medium temperature use, better room temperature hardness, yield strength, For alloys with tensile strength, impact toughness and fatigue strength, a lower solution temperature can be used to ensure a smaller grain size. During high-temperature solution treatment, various precipitated phases are gradually dissolved, and at the same time, the grains grow; during low-temperature solution treatment, not only the main strengthening phase dissolves, but also some phases may precipitate. For alloys with low supersaturation, a faster cooling rate is usually selected; for alloys with high supersaturation, cooling in air is usually used.

Solution Treatment Application:

For most non-ferrous metal alloys, the purpose of solution treatment is to obtain a supersaturated solid solution, which is prepared for the subsequent aging treatment. Figure 1 shows a schematic representation of a typical binary phase diagram with solubility changes. The room temperature equilibrium structure of C1 alloy with n-point composition is α+β two phases, α is the matrix solid solution, and β is the second phase. After the C1 alloy is heated to Tq and held for a sufficient time, the β phase will dissolve in the matrix to obtain a single-phase α solid solution. If the alloy is rapidly cooled from Tq temperature to room temperature, because the diffusion and redistribution of alloy element atoms are too late, the β phase cannot be nucleated and grown, and the β phase cannot be precipitated in the α phase. At this time, the room temperature microstructure of the alloy is n The α single-phase supersaturated solid solution of the point component (the equilibrium component of the α phase at room temperature is the b point component). This supersaturated solid solution is thermodynamically metastable, and at an appropriate temperature, the supersaturated solid solution will undergo precipitation and precipitation, thereby strengthening the alloy. The tissues after solid solution treatment are not necessarily single-phase supersaturated solid solutions. The C2 alloy shown in the figure contains the beta phase at any temperature below the eutectic temperature. When heated to Tq, the structure of alloy C2 is α-phase and β-phase of m-point composition. If it is quenched from Tq, its room temperature structure has some β phases besides the supersaturated α phase.

Solution treatment is also suitable for some alloy steels. For example, M13 high manganese steel containing 1.2%C and 13%Mn requires solution treatment. Heating it to 1050-1100°C and keeping it warm for a long enough time allows the carbide M3C to dissolve into the austenite, and then rapidly cools (water quenching) to obtain a single-phase austenite structure at room temperature. The hardness of high manganese steel with single-phase austenite structure is not high, but when it is subjected to severe impact or high pressure, its surface layer will harden rapidly, thus forming a highly wear-resistant surface layer, while the core still has good impact toughness . Based on this characteristic, M13 high manganese steel has become a widely used wear-resistant steel. Another example is 18-8 nickel-chromium stainless steel (1Cr-18Ni9, 2Cr18Ni9, etc.), the main form of heat treatment is solution treatment. Heat it to 1050-1150°C to keep it warm, and then quench it in water. A single-phase austenite structure is obtained at room temperature, which makes the material have the best corrosion resistance, high plasticity and good formability.

Solution Treatment Influence Factor:

Heating temperature, holding time and cooling rate are several main parameters that should be controlled in solution treatment.

The heating temperature can in principle be determined from the corresponding phase diagram. The upper limit temperature is usually close to the solidus temperature or the eutectic temperature. At such a high temperature the alloy has the greatest solid solubility and a fast diffusion rate. However, the temperature should not be too high, otherwise it will lead to the melting of low melting point eutectic and grain boundary phase, that is, over-burning phenomenon will occur, causing quenching cracking and reducing toughness. The minimum heating temperature should be higher than the solid solubility curve (ab line in the diagram), otherwise the performance after aging will not meet the requirements. For different alloys, the allowable heating temperature range may vary greatly. The heating temperature range of some copper alloys and alloy steels is wide, while the quenching temperature range of most aluminum alloys is very narrow, and some are even only ±5°C.

The purpose of heat preservation is to fully transform the alloy structure to the state required for quenching. The holding time mainly depends on the alloy composition, the pretreatment of the material, the original structure and the heating temperature, etc., and is also related to factors such as the amount of furnace, the thickness of the workpiece, and the heating method. The original structure is fine, the heating temperature is high, the amount of furnace charging is small, the cross-sectional size of the workpiece is small, and the holding time is short.

Rapid cooling is generally used in solution treatment. The purpose of rapid cooling is to suppress the precipitation of the second phase during the cooling process, to ensure the maximum supersaturation of solute atoms and vacancies, so as to obtain the highest strength and best corrosion resistance after aging. Water is an effective quenching medium widely used, and the cooling rate achieved by quenching in water can meet the requirements of most aluminum, magnesium, copper, nickel and iron-based alloy products. However, quenching in water is easy to cause large residual stress and deformation of the workpiece. In order to overcome this shortcoming, the water temperature can be raised appropriately, or quenched in oil, air and some special organic media. Some special quenching methods can also be used, such as austempering, graded quenching, etc.

What temperature is solution treatment?

1800 to 2450°F

Solution treating is typically performed at temperatures ranging from 1800 to 2450°F in vacuum, followed by rapid gas fan cooling to room temperature. Many materials have specific cooling rates that must be achieved to ensure the proper metallurgical microstructure is achieved in the final product.

What is solution treatment followed by?

Solution treating is typically performed in the 450 to 575°C (842 to 1067°F) range in air, followed by rapid quenching into cold water, hot water, boiling water (-T61 temper), water-polymer (glycol) solution, water spray or forced air.

Is solution heat treatment the same as annealing?

Solution annealing (also referred to as solution treating) is a common heat-treatment process for many different families of metals. Stainless steels, aluminum alloys, nickel-based superalloys, titanium alloys, and some copper-based alloys all may require solution annealing.

What is the difference between solution and precipitation heat treatment?

Solid solution strengthening involves formation of a single-phase solid solution via quenching. Precipitation heat treating involves the addition of impurity particles to increase a material’s strength.

What are the advantages of solution heat treatment?

Heat treating can improve wear resistance by hardening the material. Metals (including steel, titanium, Inconel, and some copper alloys) can be hardened either on the surface (case hardening) or all the way through (through hardening), to make the material stronger, tougher, more durable and more resistant to wear.

What is solution treatment in precipitation hardening?

The Precipitation Hardening Process

• Solution Treatment: You heat the metal to a high temperature and treat it with a solution.
• Quenching: Next, you quickly cool down the solution-soaked metal.
• Aging: Finally, you heat the same metal to a medium temperature and cool it quickly again.