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Advanced quench and temper facility for high quality bar and tube

by Pietro della Putta, Mario Fabro

The increasingly demanding market for hardened steel bars and heavy tubes for OCTG and industrial applications requires the development of more efficient quench and temper processes. TimkenSteel partnered with the SMS group to develop an advanced facility of this kind. A significant research effort was undertaken to expand the original concept of tangential water spraying with the aim of a performance increase and to make it suitable for effectively treating large bars. The paper covers the genesis, realization and initial performance of the TimkenSteel AQTF Line at the Gambrinus Steel Plant, Canton, Ohio.

It all started when TimkenSteel began investigating a new heat treatment line to expand its production at the Gambrinus plant. The year was 2014 and at that

location, close to Canton, OH, TimkenSteel was already successfully operating three other treatment lines, for tubes and bars, with capacities up to 8 t/h and 11” in diameter.

Timken Steels desires included not only a tonnage throughput increase but also to extend ambitiously the performance and technical capability of the plant to new limits. The ultimate goal was set when the company decid-ed that the new line had to process both pipes and solid bars of up to 13”, also of crack-prone grades, and that these massive diameters had to be quenched as deep as to essen-tially reaching the core.

At the same time, SMS had established a number of modern quench & temper lines, with new industry stand-ard performances, mostly dedicated to small and medium sized OCTG pipes, where tubing and casing of up to 10 ¾” were the norm, and with relatively thin wall thicknesses (20 to 30 mm at most).

Even though that background resulted in extensive experience by SMS in designing treatment furnaces as well as innovative quenching units, able to control the entire complex process parameters typical of the HTLs, a treatment line with performances requested by Timken-Steel was a completely new development and a step in uncharted territory.

The first realization was that the methods heretofore used for the quenching the OCTG tubes were not fitting the new purpose. However, the SMS Quenching Head (Fig. 1) adopted in the HTLs for the “normal size” OCTG pipes was an ideal starting point, due to the good results, and it appeared possible that a further development could exploit the full potential of some of its design concepts.

THE COMPUTATIONAL MODELSMS had already manufactured several Quenching Heads with tangential sprayer in multiple and constantly updated styles. The system repeatedly proved the superiority of the

Fig. 1: Quenching Head: A large number of calibra-ted nozzles distribute the water jets

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tangential sprayer versus other systems for obtaining a very drastic quenching speed.

A feature common to all the SMS Q.H. since its first ver-sion was the possibility to adjust the angle of the nozzle, in order to adapt to different diameter of the tubes to be processed (Fig. 2).

For quenching smaller tubes the nozzle orientation is almost radial, directing the spouts to form a very tight water tunnel close to the machine centerline, while during the processing of larger tubes the angle can be progressively opened to create a bigger tunnel, again keeping its diam-eter very close in size to that of the tube being quenched.

Empirical data seemed to indicate that heat could be removed more rapidly by bringing the water streams close to the tangent of the pipe surface. Intuitively this effect could be the result of pushing away the vapor bubbles from the surface of the hot metal thanks to the tangential speed of the water spray. These vapor surface bubbles would otherwise constitute a sort of physical barrier to the water jets while impinging the surface radially.

Aiming at a deeper understanding of the physical mech-anisms behind this phenomenon, SMS decided to start developing a computation fluid dynamics model that could possibly be possibly used to further improve the design of the quenching apparatus.

SMS started by creating a plain 2D model reproducing the geometry of the SMS Q.H. ring, and then tested dif-ferent mesh sizes and shapes to replicate the water jets coming out of the nozzles and impacting the bi-dimen-sional tube surface.

The commercial software selected for the simulation was the Ansys Fluent, having also the merit of the availa-bility of a support team for the heavy customization SMS had in mind and for creating the new algorithms necessary for the specific field of application. The initial period of the software development was spent in understanding the numerical convergence and in optimizing the mesh grid, before being able to further extend the simulation to the third dimension.

The first modeling resulted in being too heavy to be effectively handled, but it soon became clear how the geometry could be simplified by getting rid of redundan-cies. In particular, not only the longitudinal symmetry could be used, but also the periodicity along the polar coordinate.

Additional simplifications in the initial algorithm like ignoring the physical effect of gravity, as well as the movements of the tube inside of the water tunnel, i. e. its translation along the main axis and its rotation, were implemented.

During the laminar pathway, the water flow tends to stay coherent and to preserve its momentum, and therefore the geometry of the simulated model can be rationalized by reducing the gap between the nozzle and the tube surface.

The characterization of the pipe steel grade was done by means of the powerful tools incorporated in the JMat-Pro commercial software package, whose software library includes the physical parameters of the most common steel grades and, in addition, gives the possibility to cal-culate and predict the same parameters of any inputted chemistry within a vast range.

Among others, explicit characteristics like the specific heat, thermal conductivity, TTT / CCT curves and phase diagrammes are all necessary to the development of the final quenching equation.

An intermediate step of the mathematical model was the generation of the overall Heat Transfer Coefficient [W/m2h] which characterizes the cooling process and allows the computation of a complete cooling curve. The ultimate goal includes the prediction of phase transformation, i. e. typically trough a time-based function describing the con-version of the starting Austenite (C in γ-iron) into different solid phase structures, as Martensite, Bainite, Perlite and so forth.

At this point, the results obtained constituted a fully working model capable of both managing a vast array of practical situations and delivering plausible predictions. One important step was missing for the final goal: the empirical comparison with real life testing.

EXPERIMENTAL ACTIVITYHaving a fully functioning computational model SMS had to fine-tune it by collecting actual results and introduce the right correction coefficients. A testing facility was arranged in the SMS group workshop of Tarcento, (UD) in Italy, with the purpose of heating up instrumented steel pipes to observe and record the temperature behaviour throughout the cooling cycle.

Test rig descriptionThe test rig mainly included a module of SMS Quenching Head, built in full scale, placed in front of a heating oven (Fig. 3).

Fig. 2: Through a concentric rotational system, the direction of the water nozzles can be varied

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The quenching module is connected with a water pump that supplies a defined flow rate at a fixed pressure thanks to a by-pass system.

The small furnace is suitable for the reheating of tube or bar samples of any shape. When the sample has achieved a target temperature and an even distribution it is extract-ed from the furnace and immediately placed inside the quench module to avoid cooling phenomena outside of those provided by the water sprayer.In the trial described hereby, the sample was a round bar, 150 mm diameter, and with a length in accordance to the length of the surface soaked by the nozzles. The sample was equipped with five thermocouples placed at differ-ent positions. The thermocouples were K-type, grounded hot junction, linked to a data logger with an acquisition frequency of 0.3 s.

The inclination of the nozzles could be varied in order to modify the impingement angle of the water jet referred to the sample surface.

The heat transfer coefficientDuring the quenching, there are three heat transfer phe-nomena occurring at the surface:

■ Radiation of the surface towards the external environ-ment and water jets

■ Convection between water jets and sample surface ■ Boling of water droplets present on the sample surface.

Each of the three different phenomena is characterized by a transfer equation and a proper physical description. The Heat Transfer Coefficient or HTC, is a unique value that allows summarizing the effect of the three different phenomena in one single coefficient according to this relationship:

· ·S HTC T Q Q Qirr conv boilD = + +o o o

where:S: sample surface [m2]HTC: overall heat transfer coefficient [W/m2K]∆T: temperature difference between sample surface

and water jets [K]Q̇irr: thermal power exchanged by radiation [W]Q̇conv: thermal power exchanged by convection [W]Q̇boil: thermal power exchanged by boiling [W]

Due to the definition, the overall heat transfer coefficient depends on:

■ The temperature of the sample surface ■ The temperature of the water jets ■ The velocity of the water jet impinging the sample

surface ■ The impinging angle of the water jets on the sample

surface

■ The area covered by the water jets ■ The distance of the water jets from the sample surface.

Usually boiling must be avoided during quenching, as the presence of saturated steam at the sample surface inhibits the heat transfer. Hence the cooling, since it represents a barrier for the incoming water jet. This leads to the design of nozzles placed in a tangential direction in respect to the sample surface in order to help the removal of the boiling film (steam barrier).

The knowledge of the correlation between the HTC and all the parameters summarized above is important because it allows the correct design of a quenching device aimed at the metallurgical requirements that depend on the cooling rate. Once the defined type of nozzles is placed on the quenching shell the following information available:

■ Nominal flow rate and range of values that can be set ■ Geometry of the jet and velocity distribution ■ Area covered by the jets.

The distance of the nozzles from the surface to be quenched is set by the geometry of the product, accord-ing to the characteristics of the nozzles.

Two parameters remain free and can be varied, these are the impingement angle and the water flow rate.

The effect of impingement angleThe evaluation of the effect of impingement angle was accomplished by progressively modifying the inclination angle of the nozzles, in this way different water rings were obtained, as depicted in Fig. 4.

In the trial with a sample bar of Ø 150 mm, the following diameters of water rings were investigated: Ø = 80, 90, 100, 110, 120, 130, 140, 150, 155, 160, 165 and 170 mm.

The diameter Ø = 150 mm corresponds to the tangential condition. Lower diameters represent direct impact angles

Fig. 3: Front side of the test rig with visible quenching module

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while higher diameters represent jet angles that do not affect the sample surface.

All the tests were performed supplying 175 m3/h of water to the quenching head which corresponds to 10.1 l/min for each of the 288 nozzles present in the test module.

As an example, the thermal profiles measured at any impact angle with the thermocouple positioned at 37.5 mm, close to the mid-radius, is are shown in Fig. 5.

Evaluation of the overall HTCThe analysis of the test results shows that the HTC has a strong relation with the impact angle. The correlation with the water flow is also evident in Fig. 6, where it is demonstrated that after exceeding a given threshold any additional amount of water is not beneficial to the increase of the cooling rate but it rather acts as a hindrance. An explanation is that, as the water jets become more turbu-lent for the unfavourable ratio between the nozzle bore-hole and its flow, more water tends to be diverted from

the rectilinear pathway.By comparing the experimental results

with the previously CFD simulated cooling curves, it has been possible to adjust the mathematical model with some correctional factors.

The match between the two trends has already been close, with some gaps main-ly due to the temporal shift caused by the time needed to pass from the furnace to the quenching module.

Based on the comparison of the exper-imental data with the mathematical elab-oration it was possible to retrieve the rela-tionship existing between the HTC and the diameter of the water ring.

The HTC has been depicted for three different angles, with the Ø = 110, 120 and 150 mm respectively. The best Heat Transfer occurs at 120 mm of water ring diameter, a bit lower than the sample bar diameter. It is noticeable that, with the water jet impacting

Fig. 4: Schematic representing the sample to be quenched together and the water jets having different inclination

Fig. 5: Cooling rate measured with different nozzle angle: the faster rate corres-ponds to a water ring diameter slightly lower than the bar diameter

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slightly more directly at a bar of 110 mm, a significant loss of HTC occurs in a region between 200 and 600 °C, due to the lack of tangential speed which reduces the removal of the film boiling. At Ø = 150 mm the water stream partly gets off the sample surface and the HTC is already starting to lose its effectiveness.

The experimental tests, coupled with mathematical modeled results, allowed the evaluation of the relationship between the HTC and the water ring diameter. The maximum value of about 34.000 W/m2K was obtained for a water ring diameter slightly lower than the value of bar diameter, from 80 to 93 % of the value.

Out of this range the HTC value decreases because of two different causes:

■ For water ring diameter lower than 80 % of bar diameter, due to the film boiling and to unevenness of the distribution of the wet surface

■ For water ring higher than the bar diam-eter, due to the poor interaction of the water jets with bar surface.

As a final result of the experimental campaign, SMS had reset the model parameters and obtained a fully work-ing model, capable of predicting the cooling rates of the quenching machines very reliably under any configuration.

THE SMS-QUENCHING SHELLThe necessity, as identified in the first chapter, of success-fully quenching not only the relatively thin pipes but also heavy walled or even solid bars of crack sensitive grades, called for a change in paradigm on how to arrange the

entire cooling cycle. The SMS Quenching Head as described and modeled above is a progressive quench, i. e. an appa-ratus where the material is continuously advancing inside the spraying device during the cooling process: this meth-od cannot be effective where the mass of the piece, and consequently the cooling time necessary to reach its core, are both very high.

In fact the heat of the rear end of the heavy tube/bar would, by conduction, reach the front part while under the water shower, and jeopardize the spray cooling effect or even temper areas already transformed into Martensite. Both the percentage of martensitic transformation and its homogeneity throughout the stock axial direction would

Fig. 6: Cooling rate measured at 18.75 mm from the bar center, with different flows: the fastest cooling rate is at 175 m3/h

Fig. 7: “Progressive” quench process: the piece advances along the machine centerline during the cooling

Fig. 8: “Synchronous” quench concept: the piece is entirely inside the machine during the process, and it is coo-led simultaneously

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be heavily affected. Thus, the concept became the simul-taneous execution of the cooling process by means of a synchronous device, where the entire stock is subject to the spraying procedure at the same time. Still, the necessity of performing a very severe quenching suggested the oppor-tunity of maintaining as much as possible of the tangential sprayer characteristic proper of the SMS Q.H (Fig. 7).

The concept has materialized in a new device named SMS Quenching Shell (Fig. 8), which embodies the charac-teristics above in a modular machine, open at the bottom for the insertion of the pieces to be cooled, and equipped with a considerable number of spraying nozzles. All with an adjustable angle, and arranged in a semi-circle on top of the piece.

Once more the construction of a prototype and the lab-oratory trials have been preceded by the development of a computational model, representative for the new design idea.

Not only the geometry of the spraying ring had to be modified, switching from a full-circle to a top-crown config-uration, but additional physical variables had to be inserted in the calculation, in order to consider additional effects:

■ Gravity (as the system composed by machine + pro-cessed piece does no more preserve the vertical sym-metry)

■ Rotation of the piece (lacking a bottom spraying sec-tion, the tube/bar has to be rotated around its main axis, to ensure a full round water impact)

■ Coanda effect (dragging of the water film by friction).

The symmetry along the main axis of the system could be again saved to simplify the calculation. Different CFD

models had been prepared and several calculations runs launched in order to choose the most promising machine design. Each time changing the main variables such as:

■ Nozzles number (design parameter), ■ Nozzle arrangement (design parameter), ■ Water flow and speed (design parameter).

Following the choice of the design parameters above, other “process” variables had to be considered:

■ Nozzle angle (process parameter), ■ Processed piece rotational velocity (process parameter).

The above resulted in the generation of a working proto-type and, after that, of a complete unit for the TimkenSteel AQTF plant, of the before mentioned SMS Quenching Shell (Fig. 9): a machine equipped with 8 OD spraying mod-ules, covering in total a length of approx. 48” (being the max. pipe/bar length of 45”), each with about 600 nozzles and capable, in the overall, of delivering a 22,000 GPM (5,000 m3/h) of water flow. The operational working pres-sure has been set at around 43 PSI (3 bar). The crown of nozzles covers an angle, on top of the quenched sample, of about 240°.The SMS Q.S. is also equipped with an additional, larger, water nozzle positioned on one side and used for the inner cooling of the heavy tubes, whose design was made availa-ble by TimkenSteel engineering. The ID nozzle is delivering an additional 3,500 GPM (800 m3/h) at a pressure head of about 107 PSI (7.5 bar).

Once again it looked clear how the spraying angle is dra-matically influencing the cooling rate and, more in general, the overall HTC. The piece rotation is helping to overcome

Fig. 9: Front view of the SMS Q.S. in working position Fig. 10: Pipe OD 9” x WT 2” entering the furnaces (left) and inside the Shell before quenching starts (right)

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the absence of a full nozzle coverage, and the final resulting cooling rate is not too far from that of an ideal 360 °C quenching ring.

AQTF RESULTSThe extensive testing campaign performed at TimkenSteel Gambrinus AQTF towards the end of 2017 showed that the quenched test results are almost identically matching with the model theoretical forecast. It was also proven that the SMS Quenching Shell is a very severe quenching device, with both OD only and OD/ID combination of water sprayers.

Some of the testing, whose results are reported hereby as an example, is relevant to the quenching of a very thick wall pipe OD 9” x WT 2”, by using the OD sprayer only (Fig. 10). Pipes can be quenched by using both OD and ID sprayer, but the aim of this testing was the assessment of OD cooling severity.

The HRC values collected in the external surface are always above the theoretical 95 % Martensite, the mid-wall hardness corresponds to a martensitic content close to 90 %, while the internal surface shows values close to a 60 % Martensite.

CONCLUSIONUnderstanding the complex quenching phenomenona and the design of an effective device, requires a multi-dis-ciplinal methodology. The theoretical approach proved to be very successful especially when coupled with the results of extensive laboratory trials.

The resulting, corrected CFD model represented a powerful tool for the prediction of the process and the consequent development of quenching devices suitable for the specific goals.

The feedback data gained through real-life plant testing (Fig. 11) allowed the further fine-tuning of the mathemat-ical model parameters, adding precision to the computa-tional tool and enhancing its predictive capacity.

This also allows its extensive utilization for the prepara-tion of accurate working recipes, once the characteristics of the processed steel grades are known.

AUTHORS

Pietro della Putta SMS group SpA Tarcento, UD, Italy pietro.dellaputta@sms-group.com

Mario Fabro SMS group Inc. Pittsburgh, PA, U.S.A. mario.fabro@sms-group.com

Fig. 11: SMS Quenching Shell installed in the TimkenSteel Gambrinus AQTF plant