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Deformation during heating

In mechanical manufacturing, due to the potential influence of deformation on alloy selection, material source, process design and process flow, it is very important to understand the deformation mechanism. It is understandable that the discussion of deformation is usually focused on controlling the cooling of parts and the resulting stress, because it is usually regarded as the main cause of deformation in most heat treatment processes and materials. However, in many cases, heating can also cause deformation. It is usually very difficult to distinguish heating deformation from cooling deformation, but it is very important to optimize the whole heat treatment process and minimize the downstream cost.

The importance of deformation control

Deformation is an important subject in mechanical manufacturing, because it is directly related to cost. By controlling the size of deformation and the change between pipe parts, the near net shape parts can enter the optimized heat treatment process, so as to minimize the finishing allowance. Finally, the cycle time of finishing operation is minimized without significant influence on the cycle time of heat treatment. Of course, this cost saving assumption is a function of process and pipe parts, and needs to be evaluated for each case.

Understanding deformation

The deformation characteristics of pipe parts may be the size change caused by uniform microstructure change (such as phase transformation), or the shape change caused by stress caused by thermal gradient or gravity [1]. Quantifying the size and shape change of a part is often the first step to understand its deformation. Usually, this is done by measuring a considerable number of pipe parts from a statistical point of view with an appropriate level of accuracy, processing each part, and accurately measuring the post-processing of each part, and finally comparing the pre-processing and post-processing measurements. Although this method generates a lot of information and can be transformed into a process improvement strategy, it is iterative and therefore costly. Using computer modeling software with accurate material behavior data from controlled laboratory experiments can significantly reduce this iterative process, so that the initial part geometry is close to optimal from the beginning. The expansion method is an experimental method used to quantify material changes in almost all areas of the heat treatment process by accurately measuring size changes with time and temperature. The size change directly corresponds to the phase transformation behavior, which provides a means for quantitative and qualitative understanding of material behavior.

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Fig. 1: (a) length variation with temperature of hot rolled 38mnsivs5 steel and (b) hot rolled 4140 steel.
Both steels are made at the rate of 10 ° C / s to 1000 ° C. With 1 ° C / s speed control cooling to 500 ° C. Then cool to room temperature.
Figure 1 shows the thermal expansion data of two steels with the same heat treatment in the dilatometer. Although a large amount of quantitative data can be extracted from these two graphs, they will be discussed and qualitatively compared for the sake of brevity. Figure 1A shows the medium carbon vanadium (V) microalloyed steel 38mnsivs5, while figure 1b shows the common medium carbon chromium molybdenum low alloy steel 4140. The two kinds of steel were treated relatively quickly (10 ° C / s) to high austenitizing temperature (1000 C / s) ° C) , and then 1 ° C / s to 500 ° C. Then cool to room temperature. The arrow shown in the figure specifies the data recorded during heating or cooling. Generally, the material shows a nearly linear increase in length during heating and a decrease in length during cooling. The significant deviation from this nearly linear behavior indicates that the phase transition has begun. Then, when the material returns to approximately linear expansion during heating or shrinks during cooling, the phase transformation is nearly completed.
In Fig. 1a and Fig. 1b, the steel behaves similar during heating but different during cooling. During heating, due to their comparable carbon content and initial microstructure, they are similarly transformed from a mixture of ferrite and cementite (Fe3C) to austenite. In the cooling process, due to the different alloy content, the performance of the two kinds of steel is also different, resulting in 4140 steel hardening more easily, so the transformation occurs at a lower temperature. The phase transformation of 38mnsivs5 steel is reversed at relatively high temperature, and there is no net size change from the beginning to the end of heat treatment. The transformation of 4140 steel at a much lower temperature results in the formation of martensite from austenite rather than a mixture of ferrite and cementite. The difference between the beginning and end microstructure of 4140 steel after heat treatment results in the size change, which is measured by the increase of the final length of the sample. When this dimensional change occurs uniformly in the part, it can be easily explained in the change of finishing allowance. However, the size non-uniform change caused by thermal gradient will lead to shape change, which is more difficult to solve.

Causes of heating deformation

The two main factors affecting the deformation during heating are material and geometry. Unfortunately, in many cases, the root cause of heating deformation is the combined action of two factors, which may also affect the cooling deformation, thus further increasing the complexity. Finally, the heating deformation is the result of phase transformation gradient in the part. The stress generated in the process of phase transformation may exceed the local yield strength of the material, resulting in obvious shape change. Figure 2 shows the dilatometer data from the heating sections of the three materials, showing significantly different heating behaviors. The 38mnsivs5 data is the same as figure 1a, showing a comparison with Ni 200 and Ti-6Al-4V. Two kinds of nonferrous metal materials, Ni 200 and Ti-6Al-4V, represent the best and worst conditions of deformation control during heating, respectively. Ni 200 alloy is a single-phase material and does not show any phase transformation, while Ti-6Al-4V alloy shows obvious size change during heating. The deformation control of Ti-6Al-4V can be very difficult because the temperature range of phase transformation is very large (twice that of 38mnsivs5), and the size change range is five times that of 38mnsivs5. In addition to the material selection, the details of the schemes that may cause heating deformation include: Ni 200 alloy is a single-phase material and does not show any phase transformation, while Ti-6Al-4V alloy shows obvious size change during heating. In addition to material selection, details that may cause heating deformation include:

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Fig. 2: when heating 38mnsivs5 steel, commercially available pure nickel (Ni200) and ordinary titanium alloy (Ti-6Al-4V),
The transformation of length is a function of heating as a function of temperature. Titanium data from motyka and sieniawski [2].
Material uniformity: materials with low hardening rate or high alloy content may cause significant changes in chemical properties, even during isothermal holding, resulting in phase transformation gradient. Tool steel is a good example of this observation [1,3].
Part classification system: gravity can lead to deformation of thin-walled parts due to the influence of weight, resulting in yield strength exceeding a given temperature, and even distortion due to creep in some extreme cases.
Thin section: heat transfer is related to geometry. Compared with the rest of the heating process, the thin section will expand faster and the phase transition is faster. Both cases will result in stress and deformation of the part.

Reduce heating deformation

Some methods to minimize deformation during heating include slow heating rate, preheating and intermediate isothermal control. These process changes are listed in the order in which they should be performed to minimize deformation. Slow heating rate is the most important measure that can be taken to reduce deformation during heating, because it reduces the thermal gradient in the part, thus reducing the risk of deformation in all the above cases. One method that can be achieved during furnace heat treatment is to always load the cold furnace. Preheating is usually subcritical (before phase transformation during heating). For materials with significant alloy segregation or cross-section change, preheating can be combined with slow heating rate. The duration of the preheat hold usually depends on the geometry. The reason for using intermediate isothermal holding is the same as that for preheating holding, which only occurs at slightly supercritical temperature (the above phase transition occurs during heating) to make the whole cross section become single phase, and then continue heating to the final isothermal holding temperature.

Conclusion

The deformation can be minimized by controlling the thermal gradient. Although this can easily be seen as oversimplification and not applicable in all cases, it provides insight into most deformation mechanisms. Thermal and chemical composition gradients lead to phase transformation gradients, which cause stresses in the material, which may exceed the local yield strength at that temperature, resulting in shape changes. Deformation sensitive materials or parts may require a combination of slow heating, preheating and intermediate control to achieve the desired results.

By Lee rosslertner

Source: Network Arrangement – China Pipe Sleeve 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.)

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