EP1213363A1 - Method for making wear-resistant surface layers on precipitation hardenable materials - Google Patents

Abstract

Production of wear-resistant edge layers on workpieces comprises subjecting a component which has been solution annealed conventionally at a temperature Tcsa1 and heat treated conventionally at a temperature Tcpa1 to renewed short term solution annealing of the edge layer at a temperature of Tsaa Tcsa1 and a holding time of delta tssa 12 seconds; and further heat treating the inside of the component and the edge layer at a temperature of Tspa Tcpa1.

Description

The invention relates to the surface hardening of machine components. Objects, at those whose use is possible and expedient are subject to heavy wear or fatigue Components that due to high demands on the material strength at the same time high toughness are made from precipitation hardenable materials. It is particularly advantageous the invention for increasing the wear resistance of components made of stainless, precipitation hardenable martensitic steels such as B. turbine blades, pump shafts, heavy-duty bolts from the aviation industry, parts from the shipbuilding industry or special Tools can be used. Another area of application are components subject to wear high-strength, martensite-hardening (maraging) steels, which are present when high Toughness requirements cannot be used in the fully cured state.

Edge zones of components that are subject to wear but also fatigue are subject to their use significantly different loads than the component core. This fact is known to be accounted for by the fact that thermal, physical, chemical, mechanical, thermochemical or thermomechanical processes a harder, structure that is more resistant to wear and fatigue is produced than in the core, the structure of which is so it is set that it primarily addresses the existing strength and toughness requirements enough.

Without restricting generality, this background of the invention is intended in one prototypically selected, characteristic component will be explained in more detail.

Blades of low pressure stages in steam turbines are subject to during their use extremely high quasi-static (centrifugal force, blade twist), cyclical (periodic Vapor pressure, blade vibrations) and tribological (drop impact) Stresses. In particular, the constant impact of condensed water droplets to erosive wear in the vicinity of the blade leading edge. Martensithärtende 13% chrome steels are able to meet these complex demands. This will the blade material in the tempered, highly tempered state (fulfillment of the requirements for Toughness, stress crack corrosion resistance, vibration crack corrosion resistance, sufficient static and cyclical resilience; Hardness about 250 - 350 HV) and the Surroundings of the leading edge e.g. B. by flame, induction or laser beam curing Short-term hardened (very high resistance to drop impact wear, hardness approx. 390-680 HV). Increasing demands on static and cyclical resilience as well as Resistance to stress or vibration crack corrosion has recently led to Use of rustproof, precipitation hardenable martensitic steels. In contrast to The tempered steel comes with the largest share of the strength and Toughness increase not through the formation of martensite but through a targeted Precipitation heat treatment.

In addition to 10 to 20% by weight of chromium and 2-11% by weight of nickel, these steels normally contain copper (1-5% by weight) and aluminum, titanium or niobium as precipitators. A typical representative of this steel class in turbine construction is the steel X5CrNiCuNb16-4. The heat treatment usually includes at least one solution annealing at 1030 - 1080 ° C (annealing time approx. 1 h) and the actual aging treatment in the temperature range between 480 ° C and 620 ° C (time 1-4 h). The achievable mechanical parameters hardness, yield point R _p0.2 and tensile strength R _m reach their maximum at the lower limit of the conventionally possible tempering temperature of 480 ° C and decrease sharply with increasing aging temperature (see also drawing 1). So z. B. in the temperature range from 480 to 620 ° C the hardness from 425 HV to 285 HV, the yield point from 1170 to 750 MPa and the tensile strength from 1310 to 930 MPa. Due to the required toughness values, cyclical load capacities and in particular stress and vibration crack corrosion resistance, the aging temperature must be chosen so high that the 0.2% yield point and the tensile strength fall below values of around 1040 and 1000 MPa, respectively. This means that the lower range of possible tempering temperatures, which provides high hardness, cannot be used (see drawing 1).

The shortcoming of this conventional heat treatment process is therefore that the resistance to drop impact wear is too low. The reason for this lies in that the hardness at 340 - 370 HV near the surface is too low.

It is known that the surface hardness of precipitation hardenable steels is obtained by plasma nitriding can be increased to about 1000 HV [z. B. Brochure sheet from Böhler Edelstahl GmbH (Kapfenberg / Austria) on steel N700]. The lack of this procedure exists however, in that no improved drop impact resistance is achieved either. The The cause of the defect may result. a. from that the achievable nitriding depth is about 0.15 mm is too low.

Other surface layer finishing processes are also unsuitable because they are impermissibly strong in the intervene the necessary aging treatment or the achievable increase in hardness or Depth of hardness is too small.

To improve the state of the material itself, a method has become known in which a structure with a higher 0.2% yield stress and tensile strength is achieved by coupling a short-term solution annealing with a conventional aging treatment [see EE Denhard, Jr .: "Precipitation-hardenable stainless steel method and product ", U.S. Patent 3,660,176]. For this purpose, the entire semi-finished product is subjected to thorough short-term heating in a temperature range between 816 ° C. and 1149 ° C. of the solution annealing treatment and quenched within a time of 1 to 15 s by direct current passage. This is followed by a conventional aging treatment in the conventional temperature range. With a solution annealing temperature of 1149 ° C, a solution annealing time of 2 s, an aging temperature of 482 ° C and an aging time of 1 h, the 0.2% yield point from 1328 MPa to 1695 MPa and the tensile strength of 1378 MPa are achieved to increase to 1700 MPa. The hardness that can be achieved is not specified.

The shortcoming of this procedure is that it is not suitable for complicated shapes Components such as turbine blades to be used. The cause of this defect results in the geometry of the heating processes used, such as conductive or inductive heating.

Another major shortcoming is the fact that toughness and fatigue strength and in particular the stress crack and vibration crack corrosion resistance of one turbine blade treated in this way would be too small. The reason for this lies in the much too great hardness in the airfoil. If the turbine blade against it at higher temperatures would be tempered, the hardness in the area of the blade leading edge would be too low. It means that it is not possible with this process to improve the material condition itself different demands placed on the surface layer and the interior of the component, to do justice at the same time.

Another shortcoming is given by the fact that a conventional implementation the precipitation hardening the hardening ability of the short-term solution-annealed condition cannot fully use. The reason for this lies in two facts: Firstly in the fact that more hardening structural states, which cover the entire cross section of the component too little toughness can not be used and secondly in that the new metal-physical free spaces that a short-term solution annealing for the subsequent Precipitation hardening offers were previously unknown.

The aim of the invention is to provide a new and effective heat treatment process that allows components made of precipitation-hardenable materials with significantly more wear-resistant To provide edge layers without a deterioration of the other mechanical To have to accept the properties of the component.

The invention has for its object to provide a heat treatment process that it permitted, regardless of the structure and the mechanical properties of the interior of the component and without influence higher surface layer hardness up to one of the tribological load dependent, sufficiently large depth to obtain sufficient toughness, that too Complicated parts can be used and in which the aging temperature Harnessing the short-term solution-annealed condition better.

According to the invention, this object is made more wear-resistant with a method for producing Surface layers on precipitation-hardenable materials as shown in claims 1 to 14 solved.

und den Temperaturgradienten

. As described in claim 1 and / or 2, the method is based on a function optimization by a separate adjustment of the structure in the interior of the component and the surface layer. The structural state inside the component and the resulting core strength and toughness are set by a previous conventional heat treatment. Subsequently, the surface layer solution annealing according to the invention takes place in a strongly inhomogeneous temperature field, followed by an aging treatment of the entire component modified according to the invention in a homogeneous or almost homogeneous temperature field. The requirements for depth, width, position and course of the wear protection zone resulting from the analysis of the tribological and / or cyclical load distribution correspond to the desired geometry of the solution annealing zone. The solution annealing zone is generated by an outer layer heating process with sufficient power density. The depth t _{H of} the desired solution annealing zone is set by the local absorbed energy density and the local energy exposure time. The energy density and the duration of energy exposure also determine the resulting heating rate

and the temperature gradient

,

eine Kornvergröberung während der Abkühlung und eine unkontrollierte Ausscheidungshärtung. Die Angabe eines ungewöhnlich hohen Wertes für die maximale Spitzentemperatur T_{max s sa} macht von der Erkenntnis Gebrauch, dass die Härte der Randschicht, als der vorrangigen, die Verschleißbeständigkeit bei zutreffenden Verschleißarten bestimmenden Kenngröße, mit der Spitzentemperatur zunimmt oder nur wenig abfällt. Dadurch kann auch in größeren Tiefen der Kurzzeit-Lösungsglühzone ein Auflösungszustand der Ausscheidungen erreicht werden, der eine größere Einhärtetiefe oder einen flacheren Härteabfall garantiert.
Eine spezifische Ausgestaltung der Erfindung für die Klasse der martensitischen ausscheidungshärtbaren Stähle sieht Anspruch 4 vor. Durch die Wahl der erfindungsgemäßen Werte für die Spitzentemperatur T_{max s sa,}, die Temperatur T_spa und der Zeit Δt_spa wird eine deutlich höhere Randschichthärte erreicht.The choice of the two parameters as well as the holding time Δt _{s sa} and the peak temperature T _{max s sa of} the short-term solution annealing in the specified value range ensures a sufficiently rapid dissolution of the excretions without risk of grain coarsening. Depending on the peak temperature T _{max s sa} and the initial structure and the chemical composition of the material, the cooling rate according to the invention prevents

grain coarsening during cooling and uncontrolled precipitation hardening. The specification of an unusually high value for the maximum peak temperature T _{max s sa} makes use of the knowledge that the hardness of the surface layer, as the primary parameter determining the wear resistance in the case of appropriate types of wear, increases with the peak temperature or decreases only slightly. As a result, even in greater depths of the short-term solution annealing zone, the precipitates can be in a state of dissolution, which guarantees a greater depth of hardening or a shallower drop in hardness.
Claim 4 provides a specific embodiment of the invention for the class of martensitic precipitation hardenable steels. By choosing the values according to the invention for the peak temperature T _{max s sa,} the temperature T _spa and the time Δt _spa , a significantly higher hardness of the surface layer is achieved.

It is advantageous in the design of the method according to claim 10 that the residual stress state the precipitation hardened surface layer can be improved and a larger one Number of germs for the formation of fine excretions is present.

The process steps of short-term solution annealing, mechanical, can be used particularly advantageously Deformation and aging heat treatment in the further processing of semi-finished products, such as specified in claim 11 and 12, combine.

The execution of the mechanical deformation as shot peening treatment, as in claim 13 specified, can be used particularly advantageously for the optimization of the surface properties of very intricately shaped or very locally treated components, such as B. turbine blades deploy.

The heat treatment according to the invention can be carried out in various steel classes (rust and acid-resistant steels, tool steels, special steels) and steels. Such steels are z. E.g .: X5CrNiCuNb16-4 (1.4542); X2NiCoMo18-8-5 (1.6359); X2NiCoMo18-12 (1.6355); X1CrNiCoMo13-8-5 (1.6960); 17-7 PH; 17-4 PH; 15-5 PH; 17-7 B; PH 13-8Mo; PH 12-9Mo etc.

Without restricting generality, the invention is illustrated below using the example of a Complex shaped, highly stressed component made of steel X5CrNiCuNb16-4 explains:

Example:

A power amp blade made of steel N700 (factory name of Böhler Edelstahl GmbH Kapfenberg, Austria) is subject to a drop impact and is to be provided with a wear-resistant leading edge. The expected erosion zone width is 11 mm. The erosion intensity is greatest at the leading edge and decreases rapidly within the width of the erosion zone in the direction of the blade leading edge. 1.3 mm is desired as the maximum hardening depth t _{H of} the edge layer in the vicinity of the leading edge, wherein the hardening depth can decrease with increasing distance from the leading edge in accordance with the decrease in erosion intensity.

The material N700 has the following chemical target composition: carbon ≤ 0.04%; Silicon: 0.25%; Manganese: 0.40%; Chromium: 15.40%; Nickel: 4.40%; Copper: 3.30%;
Niobium: 0.30% (data in percent by weight). In order to ensure the mechanical and cyclical load capacity of the turbine blade due to centrifugal and steam power, torsion, etc., the following mechanical parameters are set by conventional heat treatment: 0.2% flow limit R _p0.2 : 930 - 1000 MPa, tensile strength R _m ≤ 1040 MPa (see dashed lines in drawing 1). For this purpose, solution _{heat treatment} is carried out at a temperature of T _csa1 = 1030 - 1060 ° C for a time of Δt = 1 h. The aging _{heat treatment} takes place at a temperature of T _cpa1 = 540 ° C - 570 ° C for a time of Δt _cpa1 = 4 h. The cooling takes place in air. The resulting micro hardness is 353 HV _0.05 and is as large in the component core as in the surface layer. This level of hardness is not sufficient for the required drop wear resistance.

The heat treatment according to the invention for producing wear-resistant surface layers is done as follows:

• Laserstrahlleistung am Auftreffort der Laserstrahlung: 2,75 kW;
• absorbierte Laserstrahlleistung: 2,2 kW;
• Vorschubgeschwindigkeit: 1000 mm/min ;
• Strahlfleckdurchmesser: 11,9 mm;
• resultierende mittlere Laserleistungsdichte: 2,0 kW/cm².

The short-term solution annealing treatment is carried out with a CO ₂ laser. For this purpose, the turbine blade is clamped in the blade chuck of a 6-axis CNC machine and moved under the laser beam at a feed rate that depends on the distance from the blade tip and is rotated at the same time. The laser beam shaping system consists of an off-axis parabolic mirror with a focal length f = 300 mm. The solution-annealing area is coated with an absorption medium 100 µm thick to increase the absorption of the CO ₂ laser radiation. A so-called auto filler with a high filler content is used as the absorbent. The parameters of the laser beam treatment are selected as follows:

• Laser beam power at the point of impact of the laser radiation: 2.75 kW;
• absorbed laser beam power: 2.2 kW;
• Feed rate: 1000 mm / min;
• Beam spot diameter: 11.9 mm;
• resulting average laser power density: 2.0 kW / cm ² .

• Aufheizgeschwindigkeit
  
• Temperaturgradient beim Aufheizen (in größerem Abstand zur Schaufelspitze)
  
• Spitzentemperatur T_{max s sa} ≈ 1350 °C;
• Haltezeit des Kurzzeit-Lösungsglühens Δt_{s sa} ≈ 0,7 s;
• Abkühlgeschwindigkeit
  

The following parameters of the short-term solution annealing result from this set of irradiation parameters:

• heating
  
• Temperature gradient during heating (at a greater distance from the tip of the blade)
  
• Peak temperature T _{max s sa} ≈ 1350 ° C;
• Holding time of the short-term solution annealing Δt _{s sa} ≈ 0.7 s;
• cooling
  

The temperature T _csa2 and holding times Δt _{csa2 of} the conventional comparative _{solution annealing treatment} would have been at T _csa2 ≈ 1050 ° C. and Δt _csa2 ≈ 1 h. Hence: T _csa2 + 300 K = T _{max ssa} .

After cooling, residual tensile stresses prevail in the solution-annealed area. Furthermore must the absorbent is removed. The absorbent is removed by a Shot peening. This ensures the reduction of the residual tensile stresses and the Development of residual compressive stresses, a portion of which also after heat treatment from aging remains.

• Auslagerungstemperatur T_spa ≈ 465 °C,
• Auslagerungszeit Δt_spa ≈ 4 h.

The subsequent aging heat treatment is carried out with the following parameters:

• _Aging temperature T _spa ≈ 465 ° C,
• Aging time Δt _spa ≈ 4 h.

The temperatures T _cpa2 and holding times Δt _{cpa2 of} the conventional comparative aging _{annealing treatment} would have been at T _cpa2 = 480 ° C and Δt _cpa2 = 1.
Hence: T _spa + 15 K = T _cpa2 ; Δt _spa = 4 * Δt _cpa2 .
The heating is carried out thoroughly in a conventional heat treatment furnace with nitrogen as a protective gas.

Drawing 2 shows the surface hardness HV _{0.05 achieved} and the hardness-depth curve. The moving average of 5 microhardness impressions is plotted. The surface hardness reaches 477 HV _0.05 . This is an increase in hardness of 124 HV _0.05 . The hardening depth up to the limit hardness of 353 HV is 1.5 mm. A significantly improved wear resistance is therefore to be expected without a significant loss in toughness of the blade. The residual compressive stress condition reached in the hardened zone reduces the stress and vibration crack corrosion susceptibility of the hardened structure.

The dependence of the microhardness HV _{0.05 of} the surface layer produced according to the invention on the aging temperature is shown in comparison in drawing 1. It can be seen that the microhardness values in the aging temperature range of 460 ° C ≤ T _spa ≤ 510 ° C are significantly higher than those of conventional heat treatment.

List of abbreviations and symbols used:

Claims (14)

Process for the production of wear-resistant surface layers on precipitation-hardenable materials by a short-term solution _annealing and a subsequent aging _{heat treatment} , characterized in that a _component which is conventionally subjected to aging _{heat treatment} and then conventionally subjected to aging _{heat treatment} at a temperature T _{cpa1 is} only the _surface layer of the component detecting short-term solution annealing at a temperature T _ssa > T _csa1 and a holding time of the short-term solution annealing Δt _ssa <12 s and then a further aging _{heat treatment is} carried out at the temperature T _spa <T _cpa1 , which encompasses both the component interior and the surface layer. A method according to claim 1, characterized in that a) the edge layer of the component is solution-annealed to a depth t _H , which corresponds to the desired hardening depth, by brief energy exposure from the component surface, b) the short-term energy effect originating from the component surface is realized by a high-energy surface layer heating process, c) the heating rate

Values of 10 ^{2 K} / _s ≤

≤ 10 ^{4 K} / _s reached, d) the temperature gradient

in the range 13 ^K / _mm ≤

≤ 1000 ^K / _{mm is} selected, e) for the peak temperature T _{max ssa of} the short-term solution annealing _treatment T _csa2 + 50 K ≤ T _{max ssa} ≤ T _csa2 +400 K, where T _{csa2 is} the conventional solution annealing _{temperature of} the corresponding material, f) the holding time of the short-term solution annealing Δ t _ssa in the temperature range in which there is a noticeable dissolution of the precipitations is in the range 10 ^-1 s ≤ Δ t _ssa ≤ 12 s, g) the cooling rate

maximum values in the cooling cycle of 5 ^K / _s ≤

≤ 10 ^{4 K} / _s reached, h) the aging heat treatment with a longer holding time .DELTA.t _spa , .DELTA.t _spa > .DELTA.t _ssh compared to the short-term solution heat _treatment and a significantly lower temperature gradient

.

<<

is carried out, i) for the temperature T _{spa of} the aging _{heat treatment} T _spa ≤ T _cpa2 ≤ T _spa + 80 K, where T _{cpa2 represents} the lower limit of the conventional aging temperature range, j) the holding time of the aging _{heat treatment} Δ t _{spa is} chosen to be one and a half to sixteen times as _long as the holding time Δ t _{cpa2 of} the conventional aging _{heat treatment} . Method according to Claim 1 or 2, characterized in that a precipitation-hardened material state is selected as the initial state for the short-term solution annealing and the subsequent aging heat treatment, the mechanical characteristics of which are chosen to be 0.2% yield strength, tensile strength and hardness according to the component stress and the aging temperature T. _cpa1 and aging time Δt _cpa1 can be set. Method according to at least one of claims 1 to 3, characterized in that the surface coating of precipitation hardenable steels with carbon contents from 0.03 to 0.08% by weight, chromium contents from 10 to 19% by weight, nickel contents from 3.0 to 11 , 0% by weight, copper contents from 1.0 to 5.0% by weight and niobium contents from 0.15 to 0.45% by weight are carried out in such a way that a) the depth t _{H of} the solution-annealed surface layer is 0.1 mm ≤ t _H <7 mm, b) for the peak temperature T _{max ssa of} the short-term solution annealing treatment 1080 ° C ≤ T _{max ssa} ≤ 1350 ° C, c) the temperature T _{spa of} the aging heat _{treatment is selected} in the range 445 ° C ≤ T _spa ≤ 500 ° C, d) the holding time of the aging heat treatment Δ t _{spa is set} in the range 1 h ≤ Δ t _spa ≤ 8 h. Method according to one of claims 1 to 4, characterized in that the high-energy boundary layer heating process is a laser heater. Method according to one of claims 1 to 4, characterized in that an electron beam heating is selected as the high-energy boundary layer heating method. Method according to one of claims 1 to 4, characterized in that an inductive edge layer heating is used as the high-energy edge layer heating method. Method according to at least one of claims 1 to 7, characterized in that the cooling rate

is achieved by external cooling. Method according to at least one of claims 1 to 7, characterized in that the cooling rate

is achieved through self-deterrence. Method according to at least one of the preceding claims, characterized in that after the short-term solution heat treatment and before the aging heat treatment, a mechanical deformation of the surface layer is carried out. A method according to claim 10, characterized in that the component is a semi-finished product and the semi-finished product is given its final shape by a deformation. A method according to claim 10 and 11, characterized in that the short-term solution heat treatment, the forming and the aging heat treatment are carried out in a continuous process. Method according to at least one of claims 10 to 12, characterized in that the mechanical deformation of the surface layer is carried out by shot peening. Method according to at least one of claims 1 to 13, characterized in that the temperature gradient

for large components in the range of 13 K / mm ≤

≤ 400 K / mm is selected.

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