Method of including features in an article manufactured from maraging stainless steel

FIELD OF THE INVENTION

This invention relates to a method of including features in an article manufactured from a maraging stainless steel, and to uses of such a method.

BACKGROUND OF THE INVENTION

In case of a conventional martensitic chromium stainless steel, including features in an article by laser melting, by focusing an electron beam or the machining process of electrical discharge machining affects material properties of a heat affected zone in a negative way. The heat affected zone, located in-between the feature and an unaffected base material of the article, loses locally its corrosion resistance. This loss of corrosion resistance is caused by local precipitation of chromium carbides, which results in a depletion of the chromium content in the matrix to a level below the corrosion resistance threshold.

Precipitation-hardening stainless steel such as Sandvik Nanoflex™ steel is used for manufacturing of articles which are used in wet and corrosion prone environment. Laser melting of specific features on Sandvik Nanoflex™ parts and laser cutting of specific hole-patterns or other shapes in Sandvik Nanoflex™ parts like rotary shaving caps and cutters and other shaving and cutting elements is an interesting machining or shaping technique compared to other conventional or unconventional machining technologies. However, laser cutting and laser melting result a loss of hardness and strength in the heat affected zones of laser melted features, in the features and at the laser cut edges.

This loss is typically restored to a proper level by giving the article a hardening heat treatment as disclosed in EP 1216311. This document discloses a method for the manufacture of steel products and products thus produced, wherein steel is subjected to isothermal martensitic formation and precipitation hardening in a martensitic structure subsequent to soft annealing and shaping. The method steps include shaping followed by solution annealing between 1050 ⁰ C and 1200 ⁰ C, quenching from the solution annealing temperature with a quenching speed of at least 5 ⁰ C per second to a temperature below

500 ⁰ C, subjecting said steel to an isothermal martensitic transformation and subsequently hardening the steel at a temperature between 450 ⁰ C and 550 ⁰ C to precipitate particles in said martensitic structure.

Restoring the properties of the heat affected zone by such a hardening treatment will actually be a re-hardening heat treatment, because the articles are often already hardened to minimize burr formation during machining and grinding upfront in a manufacturing process. However such a full (re-)hardening heat treatment can cause significant distortion of the overall initial dimensional accuracy (straightness and flatness).

SUMMARY OF THE INVENTION It is therefore advantageous to have a method for including a feature in an article without losing the dimensional accuracy of the article while restoring the hardness, strength and corrosion resistance of the heat affected zone.

A first aspect of the invention provides a method for including a feature in an article, wherein the article is manufactured from a maraging stainless steel, which method comprises the steps of: focusing an energy beam on the article at a temperature of at least 1000 ⁰ C in a nitrogen-free atmosphere, wherein the beam is configured for generating intense energy and for melting/vaporizing the maraging stainless steel in order to include the feature along with austenitic edges; cooling the article to a temperature between 0 ⁰ C and -80 ⁰ C in order to subject the austenitic edges to an isothermal martensitic transformation; and hardening at a temperature between 375 ⁰ C and 600 ⁰ C to cause particles in the edges to precipitate out from solution into martensitic structure.

The feature is created in the article by focusing an energy beam on the article in a nitrogen free atmosphere. Nitrogen is a strong austenite stabilizing element. Nanoflex™ should contain a maximum of 0.012 wt% nitrogen. Any additional pickup of nitrogen will block the transformation to martensite fully, leaving the material in the soft austenitic condition. The energy beam can melt the material and can create smoothly rounded off tips. This beam can vaporize the material and can create slots or holes in the article. After the feature is included in the article, the article is cooled to ambient temperature.

Steel subjected to a sensitizing procedure alleviates thermo -mechanical stresses which would otherwise build up internally in the article. The reduced internal thermo -mechanical stresses enable the manufacture of the article with a very accurate size and which is stable in use. Accordingly, a method for the manufacture of articles according to the invention is further characterized by subjecting the free cooled steel to an isothermal martensitic transformation by holding the steel at a temperature between 0 ⁰ C and -8O ⁰ C for at least one hour.

The final aging process step will harden the article along with the isothermally transformed martensite edges formed in the laser- melted or laser cut features. The article is pre-aged upfront the manufacturing process to minimize burr formation during conventional machining processes. Subjecting the article twice to an aging treatment is not disadvantageous, as Nanofiex™ steel is to a high degree not susceptible to over-aging. In this way the article will retain its as-received martensite content of cold rolling or original thermal hardening and also maintains its dimensional accuracy.

A method according to a preferred embodiment of the invention comprises the following steps: a. focusing the energy beam on the article at a temperature of 1400 ⁰ C to 1600 ⁰ C for a period in a range of 10 ^"7 seconds to ten seconds in the nitrogen-free atmosphere; b. cooling the article to an ambient temperature in order to sensitize the austenitic edges and thereby initiating a following isothermal martensitic transformation; c. subjecting the article to a further cooling at a temperature of between -30 ⁰ C and -50 ⁰ C for a period of at least an hour, in order to subject the austenitic edges to the isothermal martensitic transformation; and d. hardening at a temperature between 450 ⁰ C and 550 ⁰ C for at least 3 minutes to cause particles in the edges to precipitate out from solution into martensitic structure. As to step a of this embodiment, while a temperature of 1000 ⁰ C will do, the lower the temperature is below 1400 ⁰ C, the smaller the driving force for the eventual transformation from austenite to martensite. The upper boundary of the temperature range is more arbitrary, but heating above the steel melting point of 1600 ⁰ C is not relevant to the present invention. This explains a preference for the range between 1400 ⁰ C and 1600 ⁰ C.

As to steps b and c of this embodiment, it is important to reach a temperature below 0 ⁰ C within a reasonable period of time so as not to jeopardize the effectiveness of the isothermal martensite transformation. While a martensite transformation still works at 0 ⁰ C or -80 ⁰ C (though slowly, and thus requiring a longer period at which the article is kept at that temperature), it works more efficiently in the temperature range between -30 ⁰ C and -50 ⁰ C, and preferably at -40 ⁰ C.

As to step d of this embodiment, while the hardening still works at 600 ⁰ C, it appears that above 550 ⁰ C, an excessive precipitation growth (over-aging) occurs to an increasing extent, which explains the preferred upper limit of 550 ⁰ C. As regards the lower limit, while the maximum strength is obtained at temperatures above 375 ⁰ C, at temperatures above 450 ⁰ C it takes less time to achieve the desired final hardness of 500 HV or more.

According to an embodiment of the invention, the energy beam is a laser beam or an electron beam. According to another embodiment of the invention, the cooling of the article to the ambient temperature comprises the steps of: a. free cooling the article from around 1600 ⁰ C to 1300 ⁰ C; b. halting the cooling at a temperature between 1300 ⁰ C and 900 ⁰ C for at least 30 seconds in order to destabilize the austenitic edges and thereby optimizing initiation of a following isothermal martensitic transformation; c. subjecting the article to a further cooling from 900 ⁰ C to 500 ⁰ C maintaining a cooling rate of at least -5 ⁰ C /s; and d. subjecting the article to free cooling from 500 ⁰ C to ambient temperature; The article is subjected to a free cooling from 1600 ⁰ C to 1300 ⁰ C. The word

"free cooling" in the context of invention means natural cooling in a protective environment without a need for a specific rate of cooling. The advantage of the free cooling over a fast quenching is that the free cooling retains more dimensional accuracy of the article than the fast quenching. The free cooling is gentler to retain an original dimensional accuracy of the article.

The cooling temperature is halted between 1300 ⁰ C and 900 ⁰ C for at least 30 seconds. This is a sensitizing procedure and allows an initiation of the martensitic transformation to become optimal.

Later the article is subjected to a further cooling from 900 ⁰ C to 500 ⁰ C at the rate of at least -5 °C/s. This is approximately the cooling rate necessary to get the martensite transformation. Then the article is subjected to the free cooling till it reaches the ambient temperature. According to an embodiment of the invention, the melting/vaporizing is carried out in presence of a protective noble gas. An important requirement is the nitrogen- free atmosphere during the laser melting or cutting process, which may be effected by fully shielding the articles with a protective inert noble gas, like for instance Argon gas, to prevent any nitrogen pickup from air into the Nanoflex™ steel. According to a preferred embodiment of the invention, the feature includes pearls, holes, slots and/or the like. Laser melting of Sandvik Nanoflex™ parts creates pearls and laser cutting creates specific hole-patterns or other shapes in Sandvik Nanoflex™ parts like rotary shaving caps, cutters and other shaving and cutting elements.

A further object of an embodiment of the invention is to provide a method of manufacture of an article exhibiting a combination of superior strength, corrosion resistance and ductility. Such a method is further characterized in that the steel comprises chromium (Cr) in a weight percentage between 10 % and 20 %. Generally, martensitic steels with a low weight percentage of carbon, so-called maraging steels, may be with or without chromium. Corrosion resistant maraging steels comprise a weight percentage of chromium between 10.5 and 18 %. A particular type of maraging steel, which may be obtained by the method according to the invention, contains in weight percentage 10-20 % Cr, 7-10 % Ni, 3-6 % Mo, 0-9 % Co, 0.5-4 % Cu, 0.05-0.5 % Al, 0.4-1.4 % Ti and less than 0.03 % C and N.

According to a further embodiment of the invention, the above mentioned method is used for manufacturing a steel product having a homogenous hardness of at least 450 Hardness Vickers (HV).

According to a still further embodiment of the invention, the article is a cap of an electric rotary shaver. It can be a cutter of an electric rotary shaver. During hair trimming, teeth that cut the hair are in direct contact with human skin. For enhancement of the skin comfort it is beneficial to provide the teeth of the guard and the cutter with smoothly rounded off tips. An efficient method to realize this is laser melting of the tips of the trimmer teeth such that rounded pearls are formed in the front tip of the teeth. This method is suitable for many types of steel and stainless steel.

The article can be a cutter, a knife or a spring in a domestic appliance. The article may be a medical or a dental instrument. The article may also be a diaphragm plate spring in a fluid valve. In general, the article can be any domestic appliance where there is direct contact between the skin and the steel. The present invention will now be explained in greater detail with reference to the figures, in which similar parts are indicated by the same reference signs.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates loss of hardness in a laser melted pearl; Fig. 2a illustrates several laser melted pearls in row on front of trimmer teeth; Fig. 2b illustrates an exploded view of a laser melted pearl of Fig. 2a;

Fig. 3a illustrates front view of several laser melted pearls in row of trimmer teeth;

Fig. 3b illustrates an exploded front view of a laser melted pearl of Fig. 3a; Fig. 4 illustrates a shaving cap with patterns of holes and slots; and Fig. 5 illustrates cross section of two laser cut holes showing austenitic edges at an original martensitic article.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto. Any reference signs in the claims shall not be construed as limiting the scope. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

Fig. 1 is a graph plotted with hardness on y-axis and position on x-axis. The position in the graph is defined as the distance in mm between a feature included in an article and rest of the body of the article.

Fig. 2a illustrates an article 100 including laser melted pearls 110. Fig. 2b is an exploded view of the pearl 110. Fig. 3a is a front view of the article 100 including the pearls and Fig. 3b is an exploded front view of the pearl 110.

Fig. 4 illustrates an article 100 which is a shaving cap including features in the form of holes 120 and slots 130.

Fig. 5 illustrates austenitic edges 150 formed while including a feature in the article 100 with rest of the body 140 of the article 100.

It is evident from Fig. 1 that the hardness is lost in the included feature and in its vicinity. The hardness in the feature is around 300 to 330 Hardness Vickers (HV) whereas the hardness lmm away from the feature is around 600 HV. Laser cutting and laser melting result in a loss of hardness and strength in the heat affected zones of laser melted features and at the laser cut edges. This loss is caused by the back-transformation of the martensite into austenite and local precipitation of alloying elements, which results in a depletion of the content of alloying elements in the matrix. This loss is typically restored to a proper level by giving the article a hardening heat treatment.

An example of laser melting of tip of an article is explained here in detail. The article is a hair trimmer. The laser melted tips of teeth of the hair trimmer are shown in Figs. 2a, 2b, 3a and 3b. A typical hair trimmer has two cutting elements, a guard and a cutter. The guard and the cutter run in direct mechanical contact with each other. Both the guard and the cutter consist of a series of cutting teeth placed aligned in a row on a linear body. Hair is cut between flanges of the teeth of the cutter and of the guard. The trimmer cutters and guards are typically made out of cold rolled Nanofiex™ strip material with a hardness of about 300 - 400 HV. The strip or cut off pre-shapes are first aged at 500 ⁰ C for 10 minutes to harden the material to about 500-600 HV. The higher hardness makes the material more brittle and minimizes burr formation during machining and grinding in the manufacturing process. During hair trimming, teeth that cut the hair are in direct contact with human skin. For

enhancement of the skin comfort it is beneficial to provide the teeth of the guard and the cutter with smoothly rounded off tips. An efficient method to realize this is laser melting of the tips of the trimmer teeth such that rounded pearls are formed in the front tip of the teeth.

The tips are melted by focusing a laser beam on the tips of the teeth of the cutter and the guard at a temperature of 1500 ⁰ C for a period in a range of 10 ^"7 seconds to ten seconds in a nitrogen free atmosphere. The pearls are formed with austenitic edges. The trimmer is then free cooled from around 1500 ⁰ C to 1300 ⁰ C. The cooling is halted at a temperature between 1300 ⁰ C and 900 ⁰ C for at least 30 seconds in order to destabilize the austenitic edges and thereby optimizing initiation of a following isothermal martensitic transformation. Then the trimmer is subjected to a further cooling from 900 ⁰ C to 500 ⁰ C maintaining a cooling rate of at least -5 ⁰ C /s. This is approximately the cooling rate necessary to get the martensite transformation. This is rather a low cooling speed as compared to quenching. Quenching in metallurgy is associated with cooling speeds of -30 °C/s and higher up to -100°C/s or -150 °C/s. The trimmer is then subjected to free cooling from 500 ⁰ C to ambient temperature. This cooling sensitizes the austenitic edges and thereby initiates a following isothermal martensitic transformation.

The trimmer is subjected to a further cooling at a temperature between -30 ⁰ C and -50 ⁰ C for a period of at least an hour. This isothermally transforms the austenitic edges to martensitic edges. Finally the trimmer along with the laser melted pearls is hardened at a temperature between 450 ⁰ C and 550 ⁰ C for at least 3 minutes to cause particles in the edges to precipitate out from solution into martensitic structure. The final aging treatment hardens the pearls to a level of 500 HV or higher.

Figs. 4 and 5 demonstrate another example where the article is a shaving cap. The shaving caps are generally stamped out of soft annealed Nanofiex™ strip material with a thickness of 0.45 mm. The stamped caps are hardened up to a hardness of 450 - 550 HV according to the heat treatment as described in EP1216311. After hardening, the face of a shaving area and an inside running groove are machined until a membrane with a thickness of 50 - 70 μm in the shaving area remains. In this membrane a pattern of holes and slots is cut with laser as shown in Fig. 4. After laser cutting the laser cut edges are transformed to soft austenitic edges wherever the temperature exceeded 900 ⁰ C. Yet the rest of the body of the shaving cap remains martensitic as shown in Fig. 5.

The shaving cap is cooled in a similar way as explained in the above example. After cooling down from laser cutting in a nitrogen free atmosphere, the shaving cap is placed in a cooling system for 24 hours at -40 ⁰ C. In an isothermal martensite transformation

process, a martensite content of about 70 % will be reached in the edges. The hardness of the edges is at this stage about 300 HV to 330 HV. A final aging treatment does harden the isothermal martensite in the edges from this level up to a hardness level of 500 HV or higher. During laser melting and laser cutting, the articles are fully shielded with a protective inert Argon gas to prevent any nitrogen pickup from air into the Nanoflex™ steel. Nitrogen is a strong austenite stabilizing element. Nanoflex™ should contain a maximum of 0.012 wt% nitrogen. Any additional pickup of nitrogen will block the transformation to martensite fully, leaving the material in the soft austenitic condition.

A final ageing treatment is needed for both laser melted and laser cut articles. This is generally a re-hardening step. However, Nanoflex™ is to a high extent not susceptible to over- ageing. Therefore the additional ageing will not negatively affect the previously aged article either metallurgically or dimensionally. Thus a homogeneous hardness of 500 HV or higher can be reached in the article including the pearls and the area around the holes and slots. It is to be understood that although preferred embodiments, specific constructions and configurations have been discussed herein according to the present invention, various changes or modifications in form and detail may be made without departing from the scope of this invention as defined by the independent claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.