EP1213363B1 - Method for making wear resistant surface layers on articles of precipitation hardenable metallic material - 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 their use is possible and expedient, are heavily worn or fatigue-stressed Components that due to high demands on the material strength at the same time high toughness are made of precipitation hardenable materials. Is particularly advantageous the invention for increasing the wear resistance of components made of stainless, precipitation-hardenable martensitic steels, such. Turbine blades, pump shafts, heavy-duty bolts from the aerospace industry, parts from the shipbuilding industry or special ones Tools usable. Another field of application are wear-stressed components high-strength, precipitation-hardenable metallic materials, which in the presence of high Toughness requirements can not be used in the fully cured state.

Edge zones of wear-resistant but also fatigue-stressed components are subject during their use significantly different loads than the component core. This fact is known accounted for in the marginal zone by thermal, physical, chemical, mechanical, thermochemical or thermomechanical processes a harder, wear-resistant or fatigue-resistant structure is produced as in the core, its structure so is adjusted that it primarily the present strength and toughness requirements enough.

Without limiting the generality of this background of the invention to a prototypically selected, characteristic component will be explained in more detail.

Blades of low pressure stages in steam turbines are subject during their use extremely high quasi-static (centrifugal force, blade twisting), cyclic (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% chromium steels are able to meet these complex requirements. This will be the blade material in the tempered, highly tempered condition (fulfillment of the requirements of Toughness, stress corrosion cracking resistance, vibration crack corrosion resistance, sufficient static and cyclic load capacity; Hardness about 250-350 HV) and the Environment of the leading edge z. B. by means of a flame, induction or laser beam hardening short-time hardened (very high drop impact wear resistance, hardness about 390-680 HV). Increasing demands on the static and cyclic load capacity and the Resistances to stress or vibration cracking corrosion are now leading to Use of stainless, precipitation-hardenable martensitic steels. In contrast to The tempering steels comes with these the largest share of strength and Toughness increase not by the Martensitbildung but by a purposeful Ausscheidungswärmebehandlung.

These steels contain besides 10 to 20 wt.% Chromium and 2-11 wt.% Nickel usually copper (1-5 wt.%) And aluminum, titanium or niobium as precipitating agent. A typical representative of this steel class in turbine construction is the steel X5CrNiCuNb16-4. The heat treatment usually comprises at least one solution annealing at 1030-1080 ° C. (annealing time about 1 h) and the actual aging treatment in the temperature range between 480 ° C. and 620 ° C. (time 1-4 h). The achievable mechanical properties hardness, yield value 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 falls z. For example, in the temperature range from 480 to 620 ° C the hardness of 425 HV to 285 HV, the yield strength of 1170 to 750 MPa and the tensile strength of 1310 to 930 MPa. However, because of the required toughness values, cyclic load capacities and in particular stress and vibration crack corrosion resistances, the aging temperature must be set so high that the 0.2% flow limit and the tensile strength fall below values of about 1040 and 1000 MPa, respectively. This means that the high hardness-supplying lower range of possible tempering temperatures can not 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 cause lies in that the hardness at 340 - 370 HV near the surface is too small.

It is known that the surface hardness of precipitation hardenable steels by plasma nitriding can be increased up to about 1000 HV [z. B. Leaflet of Böhler Edelstahl GmbH (Kapfenberg / Austria) to steel N700]. The lack of this procedure exists however, in that even with it no improved drop impact resistance is achieved. The Cause of the defect results u. a. from that the achievable nitration depth with approximately 0,15 mm much is too low.

Other surface finishing methods are not suitable because they are inadmissibly strong in the necessary Auslagerungsbehandlung intervene or the achievable hardness increase or the Hardness depth are too low.

To improve the state of the material itself, a process has become known in which a structure with a higher 0.2% flow stress and tensile strength is achieved by coupling a short-time solution annealing with a conventional aging treatment [see EE Denhard, Jr .: "Precipitation-hardenable stainless steel method and product ", US Pat. No. 3,660,176]. For this purpose, the entire semi-finished product is subjected to the solution annealing treatment within a time of 1 to 15 s by direct current passage of a thorough short-term heating in a temperature range between 816 ° C and 1149 ° C and quenched. This is followed by a conventional aging treatment in the conventional 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, it is possible to achieve the 0.2% flow limit of 1328 MPa to 1695 MPa and the tensile strength of 1378 MPa to increase to 1700 MPa. The achievable hardness is not specified.

The shortcoming of this method is that it is not suitable for shape-complicated Components such as turbine blades to be used. The cause of this defect results in Geometry bond of the heating methods used such as conductive or inductive heating.

Another significant defect is the fact that toughness and fatigue strength and in particular the stress crack and vibration crack corrosion resistance of a turbine blade treated in this way would be too small. The cause lies in the deal too great hardness in the blade. When the turbine blade, however, at higher temperatures tempered, the hardness in the blade leading edge would be too low. It means that it is not possible with this method to improve the state of the material itself, the different requirements placed on the surface layer and the component interior, at the same time.

Another shortcoming is given by the fact that a conventional implementation the precipitation hardening the hardening ability of the short-time solution-annealed state can not fully use. The cause lies in two facts: On the one hand in that higher-hardening, the whole component cross section detecting structural conditions due Too little tenacity can not be used and, secondly, that the new ones metal-physical clearances that provide a short-term solution annealing for the subsequent Ausscheidungshärtung offers, were previously unknown.

A treatment method comprising only the surface layer for the separate microstructure adjustment in the edge layer and the component core of precipitation hardenable steels by a Surface layer heat treatment is given in SU-A-1447878. For production of Austenitic-martensitic steel sheet spring elements, the steel sheet is a conventional treatment cycle consisting of hardening, cold forming and Precipitation hardening subjected. The structure consists mainly of precipitation-hardened martensite and minor proportions of austenite. Subsequently, by a very short-term laser edge-layer annealing allows partial re-conversion of the martensite soft and ductile austenite erziehlt. This soft, mostly austenitic surface layer has a reduced risk of cracking in the spring load.

As deficient for the intended application, however, proves the Laser edge layer annealing reduced surface hardness.

The object of the invention is to provide a new and effective heat treatment process, which allows components of precipitation-hardenable materials with much more wear-resistant To provide edge layers without a deterioration of the remaining mechanical Use properties of the component to accept.

The invention has for its object to provide a heat treatment process, it regardless of structure and the mechanical properties of the component interior and without influence, higher surface hardening up to one of the tribological load to get dependent, sufficiently large depth with sufficient toughness, that too mold complicate parts can be used and in which the aging temperature Hardening possibilities of kurzzeitlösungsglimmed state better uses.

According to the invention, the above object is achieved with a method for producing wear-resistant edge layers of precipitation-hardenable materials such as in Claims 1 to 14 shown 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 component interior and the edge layer. As a first step, according to the invention, a thorough solution annealing and precipitation heat treatment are carried out in such a way that the structural state in the component interior and the resulting core strength and toughness best correspond to the later mechanical and, in particular, cyclic loading conditions. The surface layer solution annealing according to the invention then takes place in a strongly inhomogeneous temperature field, followed by an inventively modified aging treatment of the entire component in a homogeneous or nearly 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 cyclic load distribution correspond to the desired geometry of the solution annealing zone. The solution annealing zone is produced by a boundary layer heating method with sufficient power density. The depth t _{H of} the target 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 action also determine the resulting heating rate

and the temperature gradient

eine Kornvergröberung während der Abkühlung und eine unkontrollierteThe 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 fast dissolution of the precipitates 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, the wear resistance of the relevant types of wear determining characteristic increases or only slightly decreases with the peak temperature. As a result, a dissolution state of the precipitates can be achieved even at greater depths of the short-time solution annealing zone, which guarantees a greater hardening depth or a shallower hardness drop. Particularly high resistance to abrasion can be achieved if, as stated in claim 3, the parameters of the final aging heat treatment are selected so that the peak hardness of the precipitation hardness is achieved. A specific embodiment of the invention for the class of martensitic precipitation hardenable steels provides claim 4. By selecting 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 surface layer hardness is achieved.

An advantage of the method embodiment according to claim 10 is that thus the residual stress state the precipitation hardened surface layer can be improved and a larger one Number of germs for the formation of fine precipitates is present.

The process steps short-time solution annealing, mechanical Deformation and aging heat treatment in the further processing of semi-finished products, such as in claim 11 and 12, combine.

The execution of the mechanical deformation as a shot peening treatment, as in claim 13 can be particularly advantageous for the optimization of the edge layer properties of very complicated shaped or very locally treated components, such. B. turbine blades deploy.

The heat treatment according to the invention can be applied to precipitation-hardenable steels various applications (stainless and acid-resistant steels, special steels, maraging steels) deploy. Such steels are z. For example: 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 limiting the generality, the invention will be described below using the example of complicated formed, highly stressed component made of steel X5CrNiCuNb16-4:

Example:

A drop impact loaded end runner made of steel N700 (factory designation of Böhler Edelstahl GmbH Kapfenberg, Austria) should 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 erosion zone width in the direction of the blade leading edge. The maximum hardening depth t _{H of} the edge layer in the vicinity of the leading edge 1.3 mm is desired, wherein the hardening depth can decrease in accordance with the decrease in erosion intensity with increasing distance to the leading edge.

The material N700 has the following chemical composition: carbon ≤ 0.04%; Silicon: 0.25%; Manganese: 0.40%; Chromium: 15.40%; Nickel: 4.40%; Copper: 3.30%; Niobium: 0.30% (in each case in percent by weight). To ensure the mechanical and cyclic load capacity of the turbine blade due to centrifugal and Dampfkraftbeaufschlagung, twisting, etc., the following mechanical properties are set by a conventional heat treatment: 0.2% flow limit R _p0.2 : 930 - 1000 MPa, tensile strength R _m ≤ 1040 MPa (see dashed lines in Fig. 1). For this, a solution annealing _treatment is carried out at a temperature of T _csa1 = _1030-1060 ° C. for a time of Δt = 1 h. The aging _{heat treatment} is carried out at a temperature of T _cpa1 = 540 ° C - 570 ° C over a time of Δt _cpa1 = 4 h. The cooling takes place in air. The resulting microhardness is 353 HV _0.05 and is the same in the component core as in the surface layer. This level of hardness is not sufficient for the required drop impact wear resistance, but has sufficient toughness for very high cyclic loads.

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-time 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 driven under the laser beam at a feed rate which depends on the distance from the blade tip, and simultaneously rotated. The laser beam shaping system consists of an off-axis parabolic mirror with a focal length f = 300 mm. The solution-attracting area is coated to absorb absorption of CO ₂ laser radiation with an absorbent 100 microns thick. The absorbent used is a so-called autofiller with a high proportion of filler. The parameters of the laser beam treatment are chosen 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
  

From this set of irradiation parameters, the following parameters of the short-term solution annealing result:

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

The temperature T _csa2 and holding times Δt _{csa2 of} the conventional reference _{solution annealing treatment} would have been T _csa2 ≈ 1050 ° C and Δt _csa2 ≈ 1 h. Thus: T _{csa 2} + 300 K = T _{max ssa} .

After cooling, tensile residual stresses prevail in the solution-annealed area. Still must the absorbent is removed. The removal of the absorbent is carried out by a Shot peening. At the same time, this ensures the reduction of residual tensile stresses and the Build-up of residual compressive stresses, of which a proportion even after the Auslagerungswärmebehandlung persists.

• 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,
• Removal time Δt _spa ≈ 4 h.

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

Fig. 2 shows the achieved surface hardness HV _0.05 and the hardness-depth curve. In each case, the moving average of 5 microhardness impressions is plotted. The surface hardness reaches 477 HV _0.05 . That is a hardness increase of 124 HV _0.05 . The hardening depth up to the ultimate hardness 353 HV is 1.5 mm. Thus, a significantly improved wear resistance without a significant toughness loss of the blade is to be expected. The achieved compressive residual stress state in the hardened zone reduces the stress and vibration cracking susceptibility of the cured structure.
The dependence of the microhardness HV _{0.05 of} the edge layer according to the invention on the removal temperature is shown in FIG. 1 in a comparative manner. It can be seen that the microhardness values in the aging temperature range 460 ° C. ≦ T _spa ≦ 510 ° C. are significantly higher than those of the conventional heat treatment.

List of abbreviations and symbols used:

Claims (14)

1. Process for producing wear resistant surface layers on components made of precipitation-hardenable metallic materials by means of a short-time solution anneal and a subsequent age-hardening heat treatment, the component

   a) being fully solution-annealed at a temperature T_cea1,

   b) being fully subjected to an age-hardening heat treatment at a temperature T_cpa1, the temperature T_cpa1 and the holding time Δt_opa1 for the age-hardening heat treatment being set in such a way as to produce an over-aged state which is adapted to the mechanical loading on the component in terms of its mechanical characteristic variables 0.2% yield stress, tensile strength, ductility and hardness,

   c) is then subjected to a further short-time solution anneal, which affects only the surface layer of the component, at a temperature T_ssa > T_csa1 and with a holding time for the short-time solution anneal Δt_ssa < 12 s, and

   d) subsequently is exposed to a further age-hardening heat treatment, which encompasses both the interior of the component and the surface layer equally, at a temperature T_spa < T_cpa1.
2. Process according to Claim 1, characterized in that

   a) the surface layer of the component is solution-annealed down to a depth t_H, which corresponds to the desired hardness penetration depth, by means of a short-time introduction of energy starting from the component surface,

   b) the short-time introduction of energy starting from the component surface is realized by a high-energy surface-layer heating process,

   c) the heat-up rate

   

   reaches values of

   

   d) the temperature gradient

   

   is selected to be in the range

   

   e) with regard to the peak temperature T_{max ssa} for the short-time solution-annealing treatment, the following relationship applies: 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 for the short-time solution anneal Δt_ssa in the temperature range in which significant dissolution of the precipitations takes place is in the range from 10^-1s ≤ Δt_ssa ≤ 12 s,

   g) the cooling rate

   

   reaches maximum values in the cooling cycle of

   

   h) the age-hardening heat treatment is carried out with a holding time Δt_spa which is longer than the short-time solution-annealing treatment, Δt_spa > Δt_ssh and with a significantly lower temperature gradient

   

   i) the following relationship applies to the temperature T_spa of the age-hardening heat treatment T_spa ≤ T_cpa2 ≤ T_spa + 80 K, where T_cpa2 represents the lower limit of the conventional age-hardening temperature range,

   j) the holding time for the age-hardening heat treatment Δt_spa is selected to be one and a half to sixteen times the holding time Δt_cpa2 of the conventional age-hardening heat treatment.
3. Process according to Claim 1 or 2, characterized in that the final age-hardening heat treatment is carried out at a temperature T_spa and with a holding time Δt_spa which result in the desired hardness.
4. Process according to at least one of Claims 1 to 3, characterized in that the surface layer modification of precipitation-hardenable martensitic steels with carbon contents of from 0.03 to 0.08% by weight, chromium contents of from 10 to 19% by weight, nickel contents of from 3.0 to 11.0% by weight, copper contents of from 1.0 to 5.0% by weight and niobium contents of from 0.15 to 0.45% by weight is 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) the relationship 1080°C ≤ T_{max ssa} ≤ 1350°C applies to the peak temperature T_{max ssa} of the short-time solution-annealing treatment,

   c) the temperature T_spa of the age-hardening heat treatment is selected in the range from 445°C ≤ T_spa ≤ 500°C,

   d) the holding time for the age-hardening heat treatment Δt_spa is set in the range from 1h ≤ Δt_spa ≤ 8h.
5. Process according to one of Claims 1 to 4, characterized in that the high-energy surface layer heating process is a laser beam heating step.
6. Process according to one of Claims 1 to 4, characterized in that the high-energy surface layer heating process selected is an electron beam heating step.
7. Process according to one of Claims 1 to 4, characterized in that the high-energy surface layer heating process used is an inductive surface layer heating step.
8. Process according to at least one of Claims 1 to 7, characterized in that the cooling rate

   

   is achieved by means of external cooling.
9. Process according to at least one of Claims 1 to 7, characterized in that the cooling rate

   

   is achieved by self-quenching.
10. Process according to at least one of the preceding claims, characterized in that a mechanical deformation of the surface layer is carried out after the short-time solution annealing treatment and before the age-hardening heat treatment.
11. Process according to Claim 10, characterized in that the component is a semi-finished product, and the semi-finished product acquires its final shape by being worked.
12. Process according to Claims 10 and 11, characterized in that the short-time solution annealing treatment, the working and the age-hardening heat treatment are carried out as a continuous process.
13. Process according to at least one of Claims 10 to 12, characterized in that the mechanical deformation of the surface layer is carried out by means of a shot peening treatment.
14. Process according to at least one of Claims 1 to 13, characterized in that the temperature gradient

    

    for large components is selected in the range from

    

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