Nikolai I. Kobasko

Intensive quenching theory and application for imparting high residual surface compressive stresses in pressure vessel components

An alternative method for the hardening of steel parts has been developed as a means of providing steel products with superior mechanical properties through development of high residual compressive stresses on the part surface, and involves the application of intensive quenching during heat treatment. This processing method, termed Intensive Quenching, imparts high residual compressive stresses on the steel surface, thus allowing for the use of lower alloy steels, reduction or elimination of the need for carburization and shot peening, and providing for more cost-effective heat treating. Intensive quenching also provides additional environmental benefits, as the process uses plain water as the quenching media in contrast to traditional heat treatment practices which typically employ hazardous and environmentally unfriendly quenching oil. This paper presents an overview of the theory and application of intensive quenching, as well as provides experimental and computational data obtained for a variety of steel products. Also presented will be results of computer simulations of temperature; structural and stress/strain conditions for a typical pressure vessel during intensive quenching.

Real and Effective Heat Transfer Coefficients (HTCs) Used for Computer Simulation of Transient Nucleate Boiling Processes during Quenching

In this paper, real and effective heat transfer coefficients (HTCs) are considered. The real HTC is determined by dividing the heat flux density by the superheat of the liquid. The effective HTC is obtained by dividing the heat flux density by the underheat of the liquid plus the superheat of the boundary layer. In practice, especially in the heat treating industry, effective HTCs and their average values are widely used. In many cases that is not correct, because bubble formation (critical diameter) depends on the superheat ΔT = T − TS only and does not depend on the bath temperature. It is shown that due to this incorrectness, many problems arise, and misunderstanding of the quenching process often exists. So as to understand correctly the involved thermo-physical processes and receive correct data for computer simulation, in this paper, the real and effective HTCs are compared with each other. Several contradictions are shown between real and effective HTCs, such as the fact that the real HTCs do not depend on the size of a steel part and the thermal properties of a material. The effective HTC depends proportionally on the thermal properties of a material and is inversely proportional to the size of steel parts that contradict each other. In order to solve this problem, in this paper it is shown that the effective HTC is a mathematical value that can be used only for simplified cooling rate and cooling time calculations at the core of steel parts, and it cannot be used for temperature field calculations and residual stress prediction. The results of this paper can be used for computer simulation of the process of quenching and for the development of new intensive quenching technologies.

Duration of the Transient Nucleate Boiling Process and Its Use for the Development of New Technologies

In the paper a review of the transient nucleate boiling process duration is widely discussed. It has been established that duration of transient nucleate boiling process is directly proportional to square of the thickness of steel parts and inversely proportional to thermal diffusivity of a material, depends on the configuration of steel parts, liquid properties, and its velocity. The transient nucleate boiling (self-regulated thermal process) is followed by amazing regularities: The surface temperature during nucleate boiling is maintained at the level of the boiling point of the liquid, which is used as a quenchant. During this period, average effective heat transfer coefficients and average generalized Biot numbers and Kondratjev numbers can be found which significantly simplify core cooling time and cooling rate calculations. Using established characteristics of the transient nucleate boiling process, the new intensive quenching (IQ) technologies were developed: IQ-1; IQ-2: IQ-3. In the paper the steel super-strengthening phenomenon and optimal quenched layer, which provides maximal residual compressive stresses at the surface of steel parts, are discussed, which increase service life of products. Instead of oils, plain water is used as a quenchant, environmental conditions are significantly improved.

On One Nonlinear Mathematical Model for Intensive Steel Quenching and Its Analytical Solution in Closed Form

Evaluation of non-stationary nucleate boiling can be done on the basis of solving hyperbolic heat conductivity equation with the nonlinear boundary condition responsible for the process of nucleate boiling. Results of calculations can be used for modification of IQ-2 method of quenching, which is environment friendly, less expensive process, which significantly improves service life of steel parts.

One More Discussion “What is Intensive Quenching Process?”

Three intensive quenching processes known as IQ-1, IQ-2, and IQ-3 are considered. In contrast to IQ-2 and IQ-3, processes that use water as a quenchant, the two-step IQ-1 technique is actually a combination of a conventional quench (in oil or polymer/water solution) as the first step, followed by intensive quenching in water (the second step). Similar to the IQ-2 and IQ-3 methods, the IQ-1 process provides residual surface compressive stresses, optimum hardened depth, and additional strengthening (superstrengthening) of the material. A database and a method of calculating the IQ-1 process parameters are considered. The authors underlined that IQ processes are green environmentally friendly methods that significantly reduce CO2 emissions and energy consumption. Also examples of the use of IQ-2 and IQ-3 methods for quenching different kinds of steel parts (such as shafts, pinions, crosses, etc.) are provided. It is shown that the IQ technology’s rapid and uniform cooling rates increase service life of steel parts due to the creation of high residual compressive stresses at the surface of steel parts and due to additional strengthening (superstrengthening) of the material.

Cooling Capacity of Coconut Oil, Palm Oil, and a Commercial Petroleum Oil by Solving the Heat Conductivity Inverse Problem

The cooling capacity of coconut oil, palm oil, and petroleum oil were determined by solving the inverse problem (IP) using the newly developed commercial code, IQLab. It was shown that all of the oils exhibited shock-film-boiling, film-boiling, and convection-heat-transfer modes during the quenching process. The results of these investigations are necessary when developing a global database of the cooling capacity of different quenchants. The results obtained by solving the inverse problem are compared with simplified calculation results based on cooling time–temperature data obtained by using a multi-thermocouple Inconel 600 probe. The results obtained showed that the results from both methods agreed within ±10 %. These results suggest that the standard Inconel 600 probe can be used in many cases for determining average heat-transfer coefficients occurring when using vegetable oils, such as coconut oils, palm oils, and petroleum oils as quenchants.

Local Film Boiling and Its Impact on Distortion of Spur Gears During Batch Quenching

The paper discusses results of computer simulation connected with the double distortion during batch quenching of spur gears caused by a local film boiling between teeth. A carburized gear, outside diameter 2.5 in., was intensively quenched in conditions that provided heat transfer coefficient (HTC) equal to 25 000 Wm−2K−1. In some places between teeth local film boiling took place where HTC was 800 Wm−2K−1. Computer simulation showed that maximum displacement is observed between teeth where local film boiling took place. The authors came to the conclusion that increasing critical heat flux densities and elimination of local film boiling can result in decreasing distortion of spur gear. That is true for different sizes of gear during their quenching when using the second type of intensive quenching process (IQ-2) technique (a two or three-step quenching process). It is underlined that critical heat flux densities have a great effect on distortion during batch quenching. The authors also came to the conclusion that a small amount of special additives can decrease significantly distortion during quenching of gears. That is why a global database on cooling capacity of quenchants should be available which must contain critical heat flux densities of different kinds of quenchants.

Cooling Capacity of Petroleum Oil Quenchants as a Function of Bath Temperature

Other than water, the most common quenchants used for hardening steel are based on petroleum oil base stock. Cooling capacity is dependent on a number of variables including oil viscosity and additives, agitation, and bath temperature and is characterized by critical heat flux densities and heat transfer coefficients. It is shown here that there is an optimal quench oil temperature at which the critical heat flux density reaches its maximum value. Computational results are compared with results from earlier reports. These data show that the optimal quenching temperature for petroleum oils may be used to reduce the distortion of hardened steel parts. Different methods of determining critical heat flux densities and heat transfer coefficients were considered, and the most accurate results were obtained when proprietary software was used to solve an inverse problem that simultaneously yields the current and critical heat flux densities and heat transfer coefficients versus the surface temperature. The computational approach used to determine the cooling capacity of a quench oil as a function of bath temperature is discussed here.

Intensive Quenching of Steel Parts and Tools in Water Salt Solutions of Optimal Concentration

In this paper, a method of intensive quenching in water salt solutions, known as the IQ-2 process, is considered. To realize the IQ-2 process, water salt solutions were used, including with NaNO3, Na2CO3, NaCl, CaCl2, MgCl2, Ca(OH)2, and others salts. It is shown that the effectiveness of intensive quenching can be improved via the use of water salt solutions of optimal concentration. In order to prevent the corrosion of steel parts, a small amount of Ca(OH)2 was added to water salt solutions of sodium and calcium chloride to create a pH of 8 to 12. The proposed technology is less expensive compared to conventional quenching methods while providing the same or better end results in terms of increasing the service life of steel parts and tools and components made of powder metal materials. It is shown that the intensive quenching of tools by water salt solutions of optimal concentration increases the service life of tools by 1.5 to 2 times as compared with the oil quenching process.

Energy Efficient and Eco-friendly Intensively Quenched Limited Hardenability Low Alloy Steels

Low and limited hardenability (LH) steels are plain carbon steels characterized by a low content of alloying elements (Cr, Ni, Mo, W, V, etc.). The use of LH steels with an intensive quenching method allows full elimination of the carburization process for a variety of steel parts, such as, gear and bearing products, tools, low-wear parts for different applications. This is based on the steel super strengthening phenomenon and creation of high residual compressive stresses at the surface of intensively quenched steel parts. Both of these factors allow replacing expensive alloy steels with plain carbon steels. The unique characteristic of limited hardenability steels is that these alloys only harden to a shallow depth when heated and quenched. The main idea of this paper involves the creation of the optimal depth of a hardened shell which provides optimal stress distribution in quenched steel parts. Since the LH steel core does not harden significantly, a relatively high ductility of the core is maintained. The grain sizes of LH steels are greater than ASTM 8. Several patents on LH steels have been issued in Europe. A number of technical papers utilizing LH steels for gears and bearing products have been published in the Ukraine and Russia. Elimination of carburizing saves energy and prevents the emissions of thousands of tons of CO2 gases. Furthermore, the high level of compressive surface residual stresses and the steel super strengthening phenomenon eliminates the need for secondary shot peening or surface induction operations. Also, carburized alloy steels can be successfully replaced by LH steels to increase service life and decrease materials cost.