A hot forming tool and a method of forming said tool

Abstract

The invention relates to a method of producing a hot forming tool comprising the steps of: - providing a main body of a vanadium alloyed matrix steel having a hardness of 44 - 55 HRC; - providing a pre-alloyed powder of a carbon rich tool steel having a particle size distribution (PSD) within 20 – 150 µm; - pre-heating the main body to a temperature of 150 °C to 500 °C; - applying the pre-alloyed powder to at least a portion of the surface of the pre- heated main body by laser metal deposition to a pre-determined thickness, thereby obtaining a laser metal deposited material, - tempering at least once at a temperature of 500 - 650°C for at least 1h, and - machining the tempered laser metal deposited material to a final thickness, the machined laser metal deposited material having a surface hardness of 900 – 1000 HV0.2.

Description

[0001] A HOT FORMING TOOL AND A METHOD OF FORMING SAID TOOL

[0002] TECHNICAL FIELD

[0003] The invention relates to a hot forming tool such as a high-pressure die casting tool.

[0004] BACKGROUND OF THE INVENTION

[0005] High-Pressure Die Casting (HPDC) is a metal casting process in which molten metal is forced under high pressure into a closed steel die cavity. The mould cavity is created using hot work tool steel dies which have been machined into shape. HPDC offers an economical way of producing large quantities of complex, high tolerance parts in aluminium, magnesium, zinc and copper alloys. These dies are subjected to high temperatures and mechanical forces during the casting process.

[0006] Long production series are common and place tough demands on tools and materials. The increasing challenges in die cast products requires continued development of die casting alloys with higher strength and ductility, improved machinability, weldability and corrosion resistance.

[0007] The high-pressure dies can experience erosion wear. In the past, there have many attempts been made to reduce the wear by applying surface treatments such as nitriding and different types of PVD-coatings.

[0008] Vanadium alloyed matrix tool steels have been on market for decades and attained a considerable interest, because of the fact that they combine a high wear resistance with an excellent dimensional stability and because they have a good toughness.

[0009] During their lifetime high pressure dies are subjected to severe thermal cycling. Thermal fatigue is driven by larger temperature gradients and increasing number of cycles. Thermal fatigue can lead to heat checking which can be seen as a web of thermal fatigue induced cracks. This web of cracks gives an imprint on the products formed by the high-pressure dies. Hence, it would be desirable if heat checking can be reduced or eliminated.

[0010] DISCLOSURE OF THE INVENTION

[0011] The object of the present invention is to reduce or eliminate heat checking in hot forming tools by improving the resistance against thermal fatigue. Surprisingly it was found that the resistance to thermal fatigue was increased by cladding a vanadium alloyed matrix tool steel with a carbon rich tool steel. The carbon rich tool steel was added by laser metal deposition (LMD). In addition, the carbon rich tool steel provides a high surface hardness, which is beneficial for wear resistance.

[0012] The invention is defined in the claims.

[0013] DETAILED DESCRIPTION

[0014] The invention relates to a method of producing a hot forming tool comprising the steps of: providing a main body of a vanadium alloyed matrix tool steel having a hardness in the range of 44 - 55 HRC; providing a pre-alloyed powder of a carbon rich tool steel having a particle size distribution (PSD) within 20 - 150 (tin; pre-heating the main body to a temperature of 150 °C to 500 °C, preferably 200 to 400 °C; applying the pre-alloyed powder to at least a portion of the surface of the preheated main body by laser metal deposition to a pre-determined thickness, thereby providing a partly or fully laser metal deposited material on to the main body; tempering at least one time at a temperature of 500 - 650°C for at least Ih; and machining the tempered laser metal deposited material to a final thickness and thereby obtaining a hot forming tool having a surface hardness in the range of 900 - 1000 HV0.2 of the deposited material.

[0015] The produced hot forming tool comprises a main body from the vanadium alloyed matrix tool steel and the deposited material from a carbon rich tool steel which is resistant to heat checking.

[0016] The vanadium alloyed matrix steel used for the main body in the present invention has the following composition in weight % (wt. %):

[0017] C 0.2 - 0.5

[0018] Si 0.1 - 1.5, preferably 0.1 - 0.5, most preferbaly 0.1 - 0.25

[0019] Mn 0.1 - 1.0

[0020] Cr 4.0 - 6.0 Mo 1.0 - 3.0, preferably 2.0 - 2.8

[0021] V 0.2 - 1.0

[0022] Al 0.001 - 0.1

[0023] N 0.003 - 0.12 optionally one or more of Ni, Cu, Co, W and Nb in an amount of up to 1 % each, and optionally one or more of Zr, Ta, Ti and B in an amount of up to 0.1 % each, balance Fe apart from impurities.

[0024] Preferably, the deposited material is applied to the main body through LMD of a prealloyed carbon rich tool steel powder where at least 95 % of the particles having a grain size of 20 - 150 pm.

[0025] The carbon rich tool steel used for the deposited material in the present invention has the following composition in weight % (wt.%):

[0026] C+N 1.2 - 1.8

[0027] C > 0.5

[0028] Si 0.1 - 1.5

[0029] Mn 0.1 - 1.5

[0030] Cr 4.0 - 5.5

[0031] Mo + W / 2 2.5 - 4.5

[0032] W 0 - 0.5

[0033] V + Nb / 2 3.0 - 5.0

[0034] Nb 0 - 0.5

[0035] Al 0.001 - 0.1 optionally one or more of Ni, Cu, and Co in an amount of up to 1 % each, and optionally one or more of Zr, Ta, Ti and B in an amount of up to 0.1 % each, balance Fe apart from impurities,

[0036] The main body of the hot forming tool can be produced by any technique known in the art.

[0037] The main body may be subjected to conventional hardening and optionally tempering in order to give the main body a hardness in the range of 44 - 54 HRC.

[0038] Before applying the pre-alloyed powder, the main body is pre-heated to a temperature of

[0039] 150 °C to 500 °C. Crack formation at the interface between the deposited material and the main body can thereby be reduced or eliminated. By preheating the main body residual stresses and temperature gradients are reduced. Preferably pre-heated to a temperature of 200 to 400 °C, most preferably 200 to 300 °C.

[0040] The main body is, partly or fully, provided with the deposited material by use of Laser Metal Deposition (LMD), wherein a laser beam is focused onto the surface of the main body, where a melt pool is created.

[0041] The area on the main body on to which the deposited material is to be applied may optionally be machined to a predetermined depth, e.g. 0.1 - 5 mm, before laser depositing the metal on to said area.

[0042] Pre-alloyed metal powder having the desired composition is simultaneously injected into the melt pool under a protective atmosphere, thereby forming a strong metallurgical bond between the added material and the main body. The rapid solidification results in a high hardness of the deposited material, while the mechanical properties of the main body remain unaffected.

[0043] The particle size distribution of the feedstock is such that at least 95 vol. % of all particles has a size of 20 - 150 pm, preferably have more than 80 vol. % of all particles have a size of 50 - 150 pm. Preferably max 2 vol.% of the particles has a size less than 10 pm, more preferably max 1 vol.% of the particles has a size less than 10 pm.

[0044] The following restrictions may also be applied to the particle size distribution.

[0045] D10 may be > 30 pm, preferably > 40 pm, more preferable > 50 pm. D10 may be < 70 pm, preferably < 60 pm.

[0046] D50 may be 75 ±20pm, preferably 75 ±15pm, more preferably 75 ±10pm.

[0047] D90 may be > 90 pm, preferably > 100 pm. D90 may be < 130 pm, preferably < 120 pm.

[0048] The deposition is repeated layer by layer until the desired thickness has been reached. The number layers can be from one and upward. In some applications the number of layers are only a few, such as one or two or three layers. The hot forming tool is thereafter tempered at least at least once, preferably at least twice or trice, at a temperature of 500 - 650°C for at least Ih, preferably 1.5 - 3h, in order to reduce or eliminate any retained austenite.

[0049] Preferably the amount of retained austenite is reduced to below the X-ray detection limit of

[0050] 2 vol. %.

[0051] The laser metal deposited material may be machined to a final thickness and surface finish.

[0052] The hot forming tool is preferably a high-pressure die for HPDC applications. The carbon rich tool steel is preferably deposited on the surfaces facing the mould cavity formed between the two dies and / or to surfaces of the supply passages of the molten metal to the mould cavity. The hot forming tool can be any tool suitable for creating and shaping metal components that require high-temperature processes, such as forging, extrusion, or hot stamping operations.

[0053] EXAMPLE

[0054] To examine the heat checking resistance a main body in the form of a tube was produced. The tube having a length of 100 mm, an inner diameter of 35 mm and an outer diameter of 50 mm.

[0055] The tube had the following main body composition and a hardness of 45 HRC.

[0056] C 0.35

[0057] Si 0.2

[0058] Mn 0.4

[0059] Cr 5.0

[0060] Mo 2.3

[0061] V 0.6

[0062] Al 0.005

[0063] N 0.01

[0064] A full circumferential section of the tube was machined to a depth of 0.5 mm.

[0065] The tube was preheated to 350 °C, before applying a pre-alloyed powder by a conventional LMD-process to a thickness of 1 mm on to the machined section. The pre-alloyed powder had a particle size distribution meeting the requirements:

[0066] 50 < DIO < 60 66 < D50 < 84 95 < D90 < 125

[0067] The pre-alloyed powder had the following composition:

[0068] C 1.4

[0069] 51 0.4

[0070] Mn 0.4

[0071] Cr 4.7

[0072] Mo 3.5

[0073] V 3.5

[0074] Al 0.01

[0075] N 0.008

[0076] The tube with the deposited material was thereafter tempered trice at a temperature of 550 °C (3 x 2h). The overshooting portion of the laser metal deposited material was removed to a final thickness of 0.5 mm. The surface hardness of the deposited material was determined to 940 HV0.2 and no cracks could be detected upon examination.

[0077] The tube with the deposited material were thereafter subjected to a thermal fatigue test. The test setup is schematically shown in Fig. 1. The tube 1 comprises the main body 2 and the deposited material 3. Inductions coils 4 can heat the tube 1 to 680 °C through induction heating. After heating the tube 1 can be cooled by a flow of water 5 through the tube 1 and a by cooling air 6 to the exterior of the tube 1. The cooling water has a temperature of 4 °C and the tube 1 can be cooled to 80 °C in 3 seconds. The tube 1 was repeatedly heated to 680 °C and cooled to 80 °C. After 6000 cycles the deposited material was examined. The surface hardness of the deposited material was unaffected and remained at 940 HV0.2 and no cracks could be detected upon examination. The hardness of the main body at a depth of 3 mm from the surface of the tube 1, was determined to 500 HV0.2, which corresponds to 49.2 HRC. As a reference, a tube having the same main body composition, and also tempered trice at a temperature of 550 °C (3 x 2h) was subjected to the same thermal fatigue test. Upon examination, the surface hardness of the deposited material was determined to 200 HV0.2.

[0078] Cracks were also detected, and the average crack length was determined to 200 pm and the maximum crack length to 620 pm.

[0079] Hence, the deposited material was found to minimise problems due to heat checking.

[0080] Furthermore, the combination of a main body having a hardness of 49 HRC and a deposited material having a surface hardness of 940 HV0.2 resulted in a hot forming tool having a unique combination of toughness and surface hardness and thus being very suitable for high pressure die casting.

Claims

CLAIMS1. A method of producing a hot forming tool comprising the steps of: providing a main body consisting of a tool steel alloy having the following composition in weight %:C 0.2 -0.5Si 0.1 -1.5Mn 0.1 - 1.0Cr 4.0 - 6.0Mo 1.0 -3.0V 0.2 -1.0Al 0.001-0.1N 0.003-0.12 optionally one or more of Ni, Cu, Co, W and Nb in an amount of up to 1 % each, and optionally one or more of Zr, Ta, Ti and B in an amount of up to 0.1 % each, balance Fe apart from impurities, and having a hardness of 44 - 55 HRC; providing a pre- alloyed powder having the following composition in weight %:C+N 1.2- 1.8C >0.5Si 0.1 - 1.5Mn 0.1 - 1.5Cr 4.0 -5.5Mo + W / 2 2.5 - 4.5W 0-0.5V + Nb / 2 3.0 -5.0Nb 0-0.5Al 0.001-0.1 optionally one or more of Ni, Cu, and Co in an amount of up to 1 % each, and optionally one or more of Zr, Ta, Ti and B in an amount of up to 0.1 % each, balance Fe apart from impurities,and having a particle size distribution (PSD) within 20 - 150 m; pre-heating the main body to a temperature of 150 °C to 500 °C; applying the pre-alloyed powder to at least a portion of the surface of the preheated main body by laser metal deposition to a pre-determined thickness, thereby providing a partly or fully laser metal deposited material on to the main body; tempering at least once at a temperature of 500 - 650°C for at least Ih in order to reduce or eliminate any retained austenite; and machining the tempered laser metal deposited material to a final thickness, and thereby obtaining a hot forming tool having a surface hardness of 900 - 1000 HV0.2 of the laser metal deposited material.

2. The method of producing a hot forming tool as defined in claim 1, wherein the method further comprising: before applying the pre-alloyed powder to at least a portion of the surface, machining said portion to a predetermined depth, preferably said depth is in the range of 0.1 - 5 mm, more preferably 0.2 - 2 mm, most preferably 0.3 - 2 mm.

3. The method of producing a hot forming tool as defined in any one of claims 1 to 2, wherein the as-deposited material has a thickness of 1- 30 mm and wherein the final thickness of the machined deposited material is 0.3 - 29 mm.

4. The method of producing a hot forming tool as defined in any one of claims 1 to 2, wherein the deposited material is done at a layer thickness of 0.5 -1.5 mm, preferably 0.8 - 1.2 mm, and wherein the final thickness of the machined deposited material is 0.3 - 1 mm, preferably 0.4 - 0.8 mm.

5. The method of producing a hot forming tool as defined in any one of claims 1 - 4, wherein the method fulfils one at least one of the following conditions:the tempering holding time is in the range of 1 - 3 h, the steel is tempered three times.

6. The method of producing a hot forming tool as defined in any one of claims 1- 5, wherein the pre- alloyed powder has the following composition:C 1.2 - 1.6Si 0.2 - 0.9Mn 0.2 - 0.9Cr 4.5- 5.2Mo 3.0 - 4.0V 3.3 - 4.2Al 0.001 - 0.1N 0.003 - 0.12 optionally one or more of Ni, Cu, Co, W and Nb in an amount of up to 0.5 % each, and optionally one or more of Zr, Ta, Ti and B in an amount of up to 0.1 % each, balance Fe apart from impurities.

7. A hot forming tool, wherein the tool comprises a main body and, partly or fully of a laser metal deposited material, the main body consist of a tool steel alloy having the following composition in weight %:C 0.2 - 0.5Si 0.1 - 1.5Mn 0.1 - 1.0Cr 4.0 - 6.0Mo 1.0 - 3.0V 0.2 - 1.0Al 0.001 - 0.1N 0.003 - 0.12 optionally one or more of Ni, Cu, Co, W and Nb in an amount of up to 1 % each, and optionally one or more of Zr, Ta, Ti and B in an amount of up to 0.1 % each, balance Fe apart from impurities,wherein the main body has a hardness of 44 - 55 HRC and wherein the laser metal deposited material has the following composition in weight %:C+N 1.2 - 1.8C > 0.5Si 0.1 - 1.5Mn 0.1 - 1.5Cr 4.0 - 5.5Mo + W / 2 2.5 - 4.5W 0 - 0.5V + Nb / 2 3.0 - 5.0Nb 0 - 0.5Al 0.001 - 0.1 optionally one or more of Ni, Cu, and Co in an amount of up to 1 % each, and optionally one or more of Zr, Ta, Ti and B in an amount of up to 0.1 % each, balance Fe apart from impurities, and wherein the laser metal deposited material has a surface hardness of 900 - 1000 HV0.2.

8. The hot forming tool as defined in claim 7, wherein the laser metal deposited material has a thickness of 0.3 - 29 mm.

9. The hot forming tool as defined in claim 7 or 8, wherein the laser metal deposited material has a thickness of 0.5 - 1.5 mm.

10. The hot forming tool as defined in any one of claims 7 - 9, wherein the laser metal deposited material has the following composition in weight %:C 1.2 - 1.6Si 0.2 - 0.9Mn 0.2 - 0.9Cr 4.5- 5.2Mo 3.0 -4.0V 3.3 -4.2Al 0.001 -0.1N 0.003 -0.12 optionally one or more of Ni, Cu, Co, W and Nb in an amount of up to 0.5 % each, and optionally one or more of Zr, Ta, Ti and B in an amount of up to 0.1 % each, balance Fe apart from impurities.

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