FI115773B - Method for surface tempering of thermal rollers - Google Patents

Description

1,115,773

Method for surface tempering of thermal rollers

The present invention relates to a method for surface annealing of thermal rollers. One particular application is the surface hardening of a thermal roller 5 of a paper / board machine or a tumble machine.

Thermal rolls with a diameter of more than about 600 mm are typically made of die-cast cast iron. Steel rollers are relatively new in this application. In order to increase their usefulness, they must be surface hardened. Surface hardening-10 methods include induction hardening, flame hardening or laser hardening.

Induction hardening is very much used for hardening steel working rolls: - very thick tempering layers are used in these applications - layer thickness 20 - 120 mm 15 - hardening pieces diameters 300 - 700 mm - lengths of hardening pieces approx 1000 - 3000 mm (hardened part) in a quench pool - no peripheral bores - no separate ends 20 - the process generates heat so the thermal stress state is the opposite of e.g.

,. in a calender thermo roll.

> * · • · 'Induction hardening is used to harden the calender rolls in the plastics industry: • ·' · · * * rolls are used in a process similar to that of a paper machine calender rolls • * · ;;:: 25 - no water drainage · • · * - The rolls are tempered by a different method: - No separate ends on the rolls.

:.:: In general, progressive induction hardening uses preheating to maximize the depth of the hardening: · 30, to reduce the cracking tendency, especially in high carbon and high alloy steels, and to reduce the power requirement of the heater, i. Continuous quenching is used as a preheat to maintain the hardening hardness of the reinforced layer thicknesses on the outer surface while simultaneously aiming for high layer thickness and peak hardness. The surface hardness decreases as the 115773 2 heat stream flowing from inside the article increases the temperature of the hardened layer, thereby causing emissions if extinguishing is not continued for a sufficient period of time.

When thin layers are hardened, preheating and allassam-5 mud are not normally used.

Surface hardening is based on phase change austenite => martensite. The martensite reaction occurs only if the steel is cooled sufficiently quickly below the so-called Ms temperature. Under normal hardening, the piece is at about room temperature with the hardening al-10. The surface of the body is rapidly heated to the austenitic region and quenched rapidly below the Ms temperature. In this case, the phase change starts from both the surface and the inner part of the austenitic region, moving in both cases towards the middle of the hardened layer. As the piece warms up as the quenching progresses beyond the quenching point alone, the phase conversion occurs at slightly different locations throughout the section. This causes differences in the residual stress distribution between the different ends and in the microstructure within the layer itself.

Figure 1 shows the effect of cooling rate and diameter distribution on the residual stresses of a steel cylinder (Product Data Sheet: Buderus, Scmiedeprodukte, 20 Edelstahlwerke Buderus AG, 2000, Wetzlar, Germany.) This figure shows that the level of residual stress at -> 400 ° C increases sharply with increasing diameter.

* t I «> ·. * In normal surface hardening, a surface tension state is achieved as well as a surface • *. * ;; · T: Tensile stress condition less than 25 nanoseconds. As the diameter distribution increases, this

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\; * causes problems due to an increase in the stress level due to the thermal load, especially at the high residual tensile stress peak under the surface of • t '· · - *. The thermo roll syn TYY strong temperature gradients in the thermo roll internal temperature of, for example:. ·.. 300 ° C, and the treated nonwoven web of about 80 ° C, which causes tensile stresses in the thermoplastic '' 30 of the roll mantle, which together with the residual stresses generated surface hardening can in extreme cases lead toughened for disintegrating and / or peeling off the top layer.

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3,115,773

It is an object of the present invention to provide an improved method of surface annealing of large diameter thermo rolls which minimizes / eliminates the above problems.

To achieve this object, the process according to the invention is characterized in that the material of the thermo rolls is made of annealing steel, in that the process modifies the residual stress distribution resulting from hardening to substantially correspond to the or thereafter, surface quenching is carried out above temperature A3 and finally quenching, which is continued by quenching the quenched surface of the quench to a temperature below about 160 ° C with the center hole of the thermal roll and / or perforations closed during quenching and continued.

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The quenching process of the invention enhances the compression tension at the surface and reduces the tensile stress under the surface by changing the residual stress distribution due to induction hardening, flame hardening or laser hardening to better match the localized repetitive heat load and the middle part of the thermal roll. At the same time, the process is controllable, the quenching result is uniform in the longitudinal direction, and »->; . * Improve process repeatability. In particular, the invention is applicable to inductors. for use with thiocuring.

• · • ·; : 25 In the method, parameter selection is made to achieve a relatively thin • · · hardening layer less than 10 mm in accordance with the normal progressive induction hardening process • · * ·· · *. In the process, preheating is performed at or above the Ms temperature of the steel to be quenched. Due to the preheating, the required heating power of the induction equipment is lower than that of the cold piece. Tempering heating; 30 is performed above temperature A3, e.g., to about 920 ° C, and thereafter: continued extinguishing is performed. Continued quenching of the preheated tempered body through the tempered surface of the water or polymer blend removes the preheat. and during the quenching, the heat bound to the piece until the entire roll is. at the temperature of the extinguishing liquid.

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When using the process according to the invention, the austenitic layer undergoes a phase change from the cylindrical surface oriented in the radial direction to the martensitic, providing a uniform hardening layer, and the residual stress and hardness profile are also homogeneous and controlled longitudinally.

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As a result of directional cooling through the outer surface, the basic steel, which is largely a matter of cooling in continuous quenching, results in a residual compressive stress and residual tensile stress on the core portions due to a sharp drop in the outside temperature of the wall. Because of its large outer diameter and high wall strength, this stress space is much more extensive than the induction surface hardening layer located in the residual compression stress field of the surface portion. This so-called basic work. thermal quenching without phase change causes a residual compression stress state on the surface and reduces the maximum residual tensile stress due to induction hardening or other surface hardening under the surface. If extended quenching is carried out with water, as is the case with quenching, the result is a steep temperature gradient and residual stress states in the base steel which are close to the yield stress due to the large diameter of the thermal roller.

The process of preheating, induction surface hardening and. . the residual stresses of the extended quenching are added to provide a surface; higher compressive stress than the normal quenching process, and a tensile stress peak less than that of the quenched / ·; paragraph would.

25 · · · · 'Using extended quenching results in a surface layer martensite reaction to the * ···' nucleus to a better end than in conventional progressive induction hardening. This results in a higher surface layer hardness when released, and a microra · l structure with no significant residual austenite.

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The extinguishing agent may be water or dilute water-polymer solution which is: an environmentally friendly extinguishing agent.

Fig. 2 illustrates the change in the residual stress state distribution desired by the method of the invention. In the figure, the induction hardening curve shows the measured residual stress of 115773 at normal induction hardening after discharge without preheating and continued quenching. The calculated extinction curve shows the residual stress level it produces without the effect of induction hardening, and the combined stress curve shows the combined effect of preheating, induction hardening, and extended 5 extinguishing. In Figure 2, the tension value of - indicates the compressive stress and the + sign the tensile stress.

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Claims (7)

6, 115773 1. A method for surface annealing of large diameter thermo rolls, characterized in that the material of the thermo rolls is annealing steel, wherein the method modifies the residual stress distribution caused by quenching to substantially correspond to the local temperature or thereafter, surface quenching is carried out above temperature A3 and finally quenching, which is continued by quenching the quenched surface of the quench to a temperature below about 160 ° C with the center roll and / or peripheral holes of the thermal roll being closed during quenching and extensions. Method according to claim 1, characterized in that the peripheral drilling can be performed before or after surface hardening and continued quenching. Method according to one of Claims 1 to 2, characterized in that the diameter of the thermal roller is in the range of more than 600 mm to more than 1500 mm. Method according to one of Claims 1 to 3, characterized in that. . extinguishing is continued at temperatures below about 100 ° C ♦ Method according to one of Claims 1 to 4, characterized in that te- / ·; ' the carbon content of the copper is in the range of about 0.2 to about 0.7%. : 25 The method according to any one of claims 1 to 5, characterized in that the surface hardening roll in the method is a paper or board machine or a nipple thermo roll. »7 115773