SE1551093A1 - Stainless steel strip for flapper valves - Google Patents

Abstract

12 ABSTRACT The inVention relates to a cold rolled and hardened martensitic/austenitic Stainless steelstrip for flapper Valves in the compressors, Wherein the steel strip is made from steel comprising in Weight % (Wt. %): C 0.3 - 0.5Si 0.2 - 0.8Mn 0.2 - 1.0Cr 12.0 - 15.0Mo 0.50 - 2.00N 0.02 - 0.15V 0.01 - 0.20 has a matrix consisting of tempered martensite and between 5 and 15 Volume %austenite and a tensile strength (Rm) of 1970-2300 MPa.

Description

STAINLESS STEEL STRIP FOR FLAPPER VALVES TECHNICAL FIELDThe invention relates to a Stainless steel strip for flapper valves in compressors and other reed applications.

BACKGROUND OF THE INVENTION Flapper or reed valves are used in various types of applications Where a specific type ofcompression cycle is regulated for a specific purpose. It can be a refrigeration cycle in aherrnetic reciprocating compressor Working uninterrupted in a refrigerator or in the airconditioner of a car. A flapper valve is basically a spring made from a pre-hardenedsteel strip. In its simplest forrn, the flapper valve is tongue shaped, Where one end isfixed and the opposite end hangs free and regulates the liquid or gas floW in thecompressor. The flapper valve suffers from both cyclic bending stresses and cyclicimpact stresses during its service. Usually, these cyclic stresses eventually cause fatiguefailure. Accordingly, the fatigue properties are of the utmost importance for the flapper valve material.

A flapper valve made of a steel strip of this invention has its fatigue propertiesoptimized by a combined effect of modifications to the chemical composition of the steel, the non-metallic inclusions and the heat treatment.

Compressor OEMs require materials that have a higher fatigue life to improve the compressor°s performance and life.

Furthermore, there is a growing interest in the industry to develop more energy efficientand quieter compressors. The coefficient of performance (COP) can be increased byincreasing the valve lift and by reducing the thickness of the valves. Compressordesigners therefore require valve materials that have enhanced damping properties in addition to fatigue strength improvement.

The existing steel grades used for reed valves are modified versions of a carbon steelAISI 1095 and a stainless steel AISI 420 produced via conventional melting, casting,rolling and heat treatment processes. HoWever, the industry demands and resulting performance requirements mean that future flapper reeds Will increasingly need to bemade out of very thin steel strip With an increased fatigue life expectancy and higher damping properties.

DISCLOSURE OF THE INVENTIONThe general object of the present invention is to provide a pre-hardened stainless steelstrip for flapper valves having an optimized property profile such that it can be used to produce more efficient and reliable compressors.

A further object is to provide pre-hardened stainless steel strip for flapper valves, which reduces the flapper reed contribution to the overall noise levels of the compressor.

It is also an object of the present invention to provide a method of producing such an improved steel strip.

The foregoing objects, as well as additional advantages are achieved to a significantmeasure by providing a cold rolled and hardened martensitic stainless steel strip having a composition, microstructure and physical properties as set out in the claims.

The invention is defined in the claims.

DETAILED DESCRIPTION The importance of the separate elements and their interaction with each other as well asthe limitations of the chemical ingredients of the claimed alloy are briefly explained inthe following. All percentages for the chemical composition of the steel are given inweight % (wt. %) throughout the description. The amount of microstructural phases isgiven in volume % (vol. %). Upper and lower limits of the individual elements can be freely combined within the limits set out in the claims.

Carbon (0.3 - 0.5 %) is to be present in a minimum content of 0.3 %, preferably at least 0.32, 0.34, 0.36 or0.36 %. Carbon is a strong austenite stabilizer with relatively large solubility inaustenite. The upper limit for carbon is 0.5 % and may be set to 0.48, 0.46, 0.44 or 0.42%. A referred range is 0.35 - 0.41 %. In any case, the amount of carbon should becontrolled such that the amount of primary carbides of the type M23C6, M7C3 and MÖC in the steel is limited, preferably the steel is free from such primary carbides.

Silicon (0.2 - 0.8 %)Silicon is used for deoxidation. Si is a strong ferrite former and increases the carbon activity. Si is also a powerful so lid-so lution strengthening element and strengthens the steel matrix. This effect appears at a content of 0.2 %Si. A preferred range is 0.30 - 0.60%.

Manganese (0.2 - 1.0 %) Manganese is an austenite stabilizer and contributes to improving the hardenability ofthe steel. Manganese shall therefore be present in a minimum content of 0.2 %,preferably at least 0.3, 0.35 or 0.4 %. When the content of Mn is too large the amount ofretained austenite after finish annealing may be too high. The steel shall therefore contain maximum 1.0 % Mn, preferably maximum 0.8, 0.7 or 0.65 %.

Chromium (12.0 - 15.0 %) Chromium is a ferrite stabilizing element, Which is added to impart corrosion resistanceto the steel. Cr needs to be present in a content of at least 12.0 % in order to provide apassive film on the steel surface. The lower limit may be 12,4, 12, 6, 12, 8 or 13 %.

When the content of Cr exceeds 15 %, however, delta ferrite may form.

Molybdenum (0.5 - 2.0 %) Mo is a ferrite stabilizer and is known to have a Very faVourable effect on thehardenability. Molybdenum is essential for attaining a good secondary hardeningresponse. The minimum content is 0.5 % and may be set to 0.6, 0.7 or 0.8 %.Molybdenum is strong carbide forrning element and also a strong ferrite former. Themaximum content of mo lybdenum is therefore 2.0 %. Preferably Mo is limited to 1.5,1.3 or 1.1 %.

Vanadium (0.01 - 0.20 %) Vanadium forms eVenly distributed fine precipitated carbides, nitrides andcarbonitrides of the type V(N,C) in the matrix of the steel. This hard phase may also bedenoted MX, Wherein M is mainly V but other metals like Cr and Mo may be present tosome extent. X is one or both of C and N. Vanadium shall therefore be present in anamount of 0.01 - 02%. The upper limit may be set to 0.1 or 0.08 %. The lower limitmay be 0.02, 0.03, 0.04 or 0.05%.

Nitrogen (0.02 - 0.15 %) Nitro gen is a strong austenite former. N is restricted to 0.15% in order to obtain thedesired type and amount of hard phases, in particular V(C,N). Higher nitrogen contentmay lead to Work hardening, edge cracking and/or a high amount of retained austenite.

When the nitrogen content is properly balanced against the Vanadium content, Vanadium rich carbonitrides V(C,N) Will form. These Will be partly dissolved during theaustenitizing step and then precipitated during the tempering step as particles ofnanometre size. The therrnal stability of Vanadium carbonitrides is considered to bebetter than that of Vanadium carbides. Therefore the resistance against grain growth athigh austenitizing temperatures is enhanced. The lower limit may be 0.02, 0.03, 0.04 or0.05 %. The upper limit may be 0.12, 0.10, 0.08 or 0.06 %.

Nickel (S 2.0 %) Nickel is an austenite former. Ni may be present in an amount of 52.0 %. It gives thesteel a good hardenability and toughness. However, because of the expense, the nickelcontent of the steel should be limited. The upper limit may therefore be set to 1.0, 0.5 or0.5%. However, Ni is norrnally not deliberately added.

Cobalt (í 2.0 %) Cobalt is an austenite former. Co causes the solidus temperature to increase andtherefore provides an opportunity to raises the hardening temperature. Duringaustenitization it is therefore possible to dissolve larger fraction of carbides and therebyenhance the hardenability. Co also increases the MS temperature. However, largeamount of Co may result in a decreased toughness and Wear resistance. The maximumamount is 2 % and may be set to 0.5 %. However, for practical reasons, such as scrap handling, a deliberate addition of Co is norrnally not made.

Copper (S 2.0%) Cu is an austenite stabilizing element but has a loW solubility in ferrite. Cu maycontribute to increasing the hardness and the corrosion resistance of the steel. However,it is not possible to extract copper from the steel once it has been added. This drasticallymakes the scrap handling more difficult. For this reason, the upper limit may be 1.0, 0.5, or 0.3 %. Copper is norrnally not deliberately added.

Aluminium (í 0.06 %) Aluminium may be used for deoxidation in combination With Si and Mn. The lowerlimit is set to 0.001, 0.003, 0.005 or 0.007% in order to ensure a good deoxidation. Theupper limit is restricted to 0.06% for avoiding precipitation of undesired phases such asAlN and hard, brittle Alumina inclusions. The upper limit may be 0.05, 0.04, 0.03, 0.02or 0.015 %.

Tungsten (S 2 %) In principle, mo lybdenum may be replaced by twice as much With tungsten because oftheir chemical similarities. However, tungsten is expensive and it also complicates thehandling of scrap metal. The maximum amount is therefore limited to 2 %, preferably 0.5 % or 03 % and most preferably no deliberate additions are made.

Niobium (S 0.05%) Niobium is similar to vanadium in that it forms carbonitrides of the type M(N,C) andmay in principle be used to replace part of the vanadium but that requires the doubleamount of niobium as compared to vanadium. However, Nb results in a more angularshape of the M(N,C) and these are also much more stable than V(C,N) and maytherefore not be dissolved during austenitising. The maximum amount is therefore 0.05%, preferably 0.01 % and most preferably no deliberate additions are made.

Ti, Zr and Ta (S 0.05% each)These elements are carbide forrners and may be present in the alloy in the claimedranges for altering the composition of the hard phases. However, norrnally none of these elements are added.

Boron (S 001%)B may be used in order to further increase the hardness of the steel. The amount islimited to 0.0l%, preferably S 0.005 or even S 0.001 %.

Ca and REM (Rare Earth Metals)These elements may be added to the steel in the claimed amounts in order to further improve the hot Workability and to modify the shape of non-metallic inclusions.

Impurity elements P, S and O are the main impurities, Which have a negative effect on the mechanicalproperties of the steel strip. P may therefore be limited to 003%, preferably to 0.0l%. Smay be limited to 003, 0.01, 0008, 00005 or 0.0002%. O may be limited to 0003,0.002 or 0.00l%.

The present inventors have systematically investigated the effect of a modified chemicalcomposition and a modified heat treatment on the mechanical properties of the flappervalve material. The modifications made to the chemical composition relative to the conventional material Were mainly focused on increases in nitrogen and vanadium although some benefits were also gained from increases in austenite levels and tighter control over such elements as carbon, manganese and phosphorus.

The continuous hardening of valve strip was undertaken using different fumaceparameters to map the hardening response of material from the conventional andmodified chemical compositions. The production trials were carried out at a constantline speed with hardening temperatures in the range from 1000 °C to 1080°C,quenching into a molten lead alloy at a temperature in the range of 250°C to 350°C and tempering at temperatures in the range from 220°C to 600°C.

The mechanical properties resulting from these hardening trials on conventionalmaterial corresponded to:0 a yield strength Rpog range between 1300 MPa and 1600 MPa,0 a tensile strength Rm range between 1740 MPa and 2100 MPa 0 an elongation A50 range between 4 % and 6 % Further continuous hardening trials were carried out on material with the modifiedchemical composition and non-metallic inclusion content. The production trials werecarried out at a constant line speed with hardening temperatures in the range from 1050°C to 1100°C, quenching into a molten lead alloy at a temperature in the range of 250°C to 350°C and tempering at temperatures in the range from 220°C to 600°C.

The mechanical properties resulting from further hardening trials on material with the modified chemical composition and non-metallic inclusion content corresponded to: 0 a Rpog range between 1400 MPa and 1750 MPa,0 a Rm range between 1970 MPa and 2300 MPa0 a A50 range between 4 % and 8 % EXAMPLEIn this example a stainless steel strip according to the invention is compared to a conventional stainless steel strip. The composition of the investigated steels was as follows:Conventional InventiveC 0.38 0.40Si 0.36 0.42 Mn 0.48Cr 13.1Mo 0.98N 0.017V 0.009Ni 0.31 P 0.018S 0.0004 Fe and impurities balance. 0.5613.40.990.0520.0550.150.0180.0006 The cold ro11ed strips used for the hardening and tempering tria1s a11 had a thickness of 0.203 mm and a Width of 140 mm. The strips Were subjected to hardening and tempering in the above mentioned continuous hardening fumace. Tensi1e strength measurements Were made according to ISO 6892:2009. Fig. 1 disc1oses tensi1e properties as a function of the austenitising temperature. Fig. 2 disc1oses the tensi1e properties as a function of the tempering temperature.

INDUSTRIAL APPLICABILITY The inventive stee1 strip can be used for producing flapper va1ves for compressors having improved properties.

Claims (8)

1. A cold rolled and hardened martensitic/austenitic Stainless steel strip for flapperValves in the compressors, Wherein the steel strip a) is made from steel consisting of in Weight % (Wt. %): C 0.3 - 0.5Si 0.2 - 0.8Mn 0.2 - 1.0Cr 12.0 - 15.0Mo 0.5 - 2.0N 0.02 - 0.15V 0.01 - 0.20Ni S 2.0 Co S 2.0 Cu S 2.0 W S 2.0 Al S 0.06 Ti S 0.05 Zr S 0.05 Nb S 0.05 Ta S 0.05 B S 0.01 Ca S 0.009REM S 0.2 Fe and impurities balance, b) has a matrix consisting of tempered martensite and between 5 and 15Volume % austenite, c) has a tensile strength (Rm) of 1970-2300 MPa, d) has a thickness of 0.07- 3 mm and a width of S 500 mm.

2. A strip according to claim 1 fulfilling at least one of the following requirements: VNiCoCuWAlTiZrNb TaBCa and wherein the impurity contents of P, S and O fulf1ls the following requirementsPSO

3. A strip according to claim 1 or 2 fi1lf1lling the following requirements: 0.35 - 0.410.30 - 0.600.40 - 0.6513 - 140.8 - 1.20.03 - 0.130.02 - 0.10S 0.5 S 0.5 S 0.5 S 0.5 S 0.01 S 0.01 S 0.01 S 0.01 S 0.01S 0.0010.0005 - 0.002 S 0.03S 0.03S 0.003 C 0.35 - 0.41Si 0.30 - 0.60Mn 0.40 - 0.65Cr 13 - 14 Mo 0.8 - 1.2 N 0.03 - 0.10V 0.03 - 0.09

4. A strip according to any of the preceding claims, fulf1lling at least one of the following requirements: a tensile strength (Rm) of 2000-2200 MPa, a yield strength (Rpoq) of 1500-1750 MPa, a Vickers Hardness (HV1) of 570 - 650, a ductility A50 of 4-9 %.

5. A strip according to any of the preceding claims, fulf1lling the following requirements : reverse bending fatigue is > 850 MPa

6. A strip according to any of the preceding claims having a thickness of 0.1 - 1.5 mm and/or a Width of5 - 150 mm.

7. A strip according to any of the preceding claims, Wherein the maximum globular inclusion size is 6 um.

8. 10. 11. 12. 11 A strip according to any of the preceding claims, Wherein the primary inclusionspecies are Silicate type With a maximum width of 4 um. A method of producing a strip according to any of claims 1-8, Wherein the method comprises the following steps: a) Hot rolling a steel having a composition as defined in any of claims 1-3,b) Cold rolling the hot rolled strip to a thickness of 0.07-3mm, c) Continuously hardening and tempering the cold rolled strip, d) Optionally, slitting the cold rolled strip. A method according claim 9, Wherein the austenitising temperature is 1000 - 1150 °C in step c) and Wherein tempering temperature is 200 - 600 °C. A method according claim 9 or 10, Wherein the hardening involves quenchingthe strip in a bath of mo lten lead or lead alloy, the bath preferably holding atemperature of 250 - 350 °C. A method according to any of claims 9-11, Wherein the steel used is produced bypoWder metallurgy and Wherein the maximum globular inclusion size of said steel is 6 um.