JP2010229524A

JP2010229524A - Nitriding quenching method

Info

Publication number: JP2010229524A
Application number: JP2009080445A
Authority: JP
Inventors: Takuya Shimizu; 拓也清水; Izuru Yamamoto; 出山本
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2009-03-27
Filing date: 2009-03-27
Publication date: 2010-10-14

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitriding quenching method which removes fluctuation of nitriding concentration of nitrogen permeating and diffusing into a work and, moreover, can stabilize quality of a nitrided quench article. <P>SOLUTION: The nitriding quenching method of causing nitrogen to permeate and diffuse into the work 1 made of steel material and, thereafter, performing quenching includes: a plastic flow layer improving process S1 of raising a temperature of the work 1 to the austenite region temperature of the work 1 and retaining the work 1 for a prescribed time as it is to improve a plastic flow layer of the work; a nitriding process S2 of raising or lowering the temperature of the work 1 to the nitriding temperature and nitriding the work 1 of which the plastic flow layer is improved; and a quenching process S3 of quenching the work 1 subjected to nitriding. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

本発明は、鉄鋼素材からなるワークに窒素を浸透拡散させ、その後これに焼入れを行う、浸窒焼入処理方法の技術に関する。 The present invention relates to a technique of a nitriding and quenching treatment method in which nitrogen is permeated and diffused into a work made of a steel material, and then quenched.

従来、鉄鋼素材からなるワークの表面に窒素を浸透拡散させ、その後、これに焼入れを行って、当該ワークの表面に硬化層が形成された浸窒焼入処理品を得ることは公知となっている。例えば、特許文献１に示す如くである。 Conventionally, it is publicly known that nitrogen is permeated and diffused on the surface of a workpiece made of a steel material, and thereafter quenching is performed to obtain a nitriding and quenching treated product having a hardened layer formed on the surface of the workpiece. Yes. For example, as shown in Patent Document 1.

このような浸窒焼入処理品を得るワークの浸窒焼入処理方法は、まず、鉄鋼素材からなるワークを密閉された処理炉内に配置する。次いで、図３（ａ）に破線で示す如く、処理炉内の温度を浸窒処理温度、例えば８００℃に昇温させる。さらに、ワークの温度を前記浸窒処理温度まで上昇して当該浸窒処理温度を保持しつつ、処理炉内にアンモニアガスを単独でまたは浸窒処理に影響を与えないガスとともに導入して、ワークの表面から窒素を浸透拡散させる。そして、窒素が浸透拡散されたワークを油急冷等することにより焼入れを行う。このようにして、ワークに浸窒焼入処理を行って、ワークの表面に硬化層が形成された浸窒処焼入理品を得る。
また、このような鉄鋼素材からなるワークは、浸窒焼入処理の前工程において切断や切削等の加工が施されるため、ワークの加工面の表層部分には、加工抵抗による塑性流動層が存在する。 In the method of nitriding and quenching a workpiece for obtaining such a nitriding and quenching processed product, first, a workpiece made of a steel material is placed in a sealed processing furnace. Next, as shown by a broken line in FIG. 3A, the temperature in the processing furnace is raised to a nitriding temperature, for example, 800 ° C. Further, the temperature of the workpiece is increased to the nitriding treatment temperature and the nitriding treatment temperature is maintained, and ammonia gas is introduced into the treatment furnace alone or together with a gas that does not affect the nitriding treatment. Nitrogen penetrates and diffuses from the surface. And it quenches by carrying out oil quenching etc. of the workpiece | work in which nitrogen was osmose | permeated and diffused. In this manner, the workpiece is subjected to nitriding and quenching to obtain a nitriding and quenching cured product having a hardened layer formed on the surface of the workpiece.
In addition, since a workpiece made of such a steel material is subjected to processing such as cutting and cutting in the pre-nitrogen quenching process, a plastic fluidized layer due to processing resistance is formed on the surface layer portion of the workpiece processing surface. Exists.

特開２００７−４６０８８号公報JP 2007-46088 A

しかしながら、塑性流動の程度は、ワークを加工する際の加工条件によって異なる。つまり、ワークの加工面の表層部分に存在する塑性流動層の状態は、例えば、ワークを加工する際に用いられる刃具の状態や当該刃具の接触条件等によって異なる。
さらに、塑性流動が大きい程、当該塑性流動層における粒界密度は大きくなるため、当該粒界密度が大きいワークの表面からは、浸窒処理における窒素の浸透拡散が進行し易くなる。他方、塑性流動が小さい程、当該塑性流動層における粒界密度は小さくなるため、当該粒界密度が小さいワークの表面からは、浸窒処理における窒素の浸透拡散が進行し難くなる。
このため、鉄鋼素材からなるワークに浸窒焼入処理を行った場合、塑性流動層の状態によって、ワークに浸透拡散された窒素の浸窒濃度はバラつき、ひいては浸窒焼入処理品の品質にバラつきが生じることとなる。 However, the degree of plastic flow varies depending on the processing conditions when processing the workpiece. That is, the state of the plastic fluidized bed existing in the surface layer portion of the work surface of the workpiece varies depending on, for example, the state of the blade used when machining the workpiece, the contact condition of the blade, and the like.
Furthermore, since the grain boundary density in the plastic fluidized bed increases as the plastic flow increases, the permeation and diffusion of nitrogen in the nitriding process easily proceeds from the surface of the workpiece having the large grain boundary density. On the other hand, the smaller the plastic flow, the smaller the grain boundary density in the plastic fluidized bed. Therefore, it is difficult for nitrogen to penetrate and diffuse in the nitriding treatment from the surface of the workpiece having the lower grain boundary density.
For this reason, when nitrogen-quenching treatment is performed on workpieces made of steel materials, the nitrogen concentration of nitrogen that has penetrated and diffused into the workpiece varies depending on the state of the plastic fluidized bed, resulting in the quality of the nitrogen-quenched products. Variations will occur.

本発明は以上のような状況に鑑みてなされたものであり、ワークに浸透拡散された窒素の浸窒濃度のバラつきをなくし、ひいては浸窒焼入処理品の品質を安定させる浸窒焼入処理方法を提供することを課題とする。 The present invention has been made in view of the situation as described above, and eliminates the variation in the nitrogen concentration of nitrogen that has permeated and diffused into the workpiece, which in turn stabilizes the quality of the nitrogen-quenched product. It is an object to provide a method.

本発明の解決しようとする課題は以上の如くであり、次にこの課題を解決するための手段を説明する。 The problem to be solved by the present invention is as described above. Next, means for solving the problem will be described.

即ち、請求項１においては、鉄鋼素材からなるワークに窒素を浸透拡散させ、その後これに焼入れを行う、浸窒焼入処理方法であって、該ワークの温度を該ワークのオーステナイト域温度に昇温して所定時間保持することによって、該ワークの塑性流動層を改良する、塑性流動層改良工程と、前記ワークの温度を浸窒処理温度に昇温または降温して、塑性流動層が改良された前記ワークに浸窒処理を行う、浸窒処理工程と、浸窒処理が行われた前記ワークに焼入処理を行う、焼入処理工程と、を具備するものである。 That is, according to claim 1, a nitrogen-quenching quenching method in which nitrogen is infiltrated and diffused into a work made of a steel material and thereafter quenched, and the temperature of the work is raised to the austenite region temperature of the work. The plastic fluidized bed is improved by raising and lowering the temperature of the workpiece to the nitriding temperature by improving the plastic fluidized bed of the workpiece by heating and holding for a predetermined time. A nitriding treatment process for performing a nitriding treatment on the workpiece, and a quenching treatment step for performing a quenching treatment on the workpiece subjected to the nitriding treatment.

本発明の効果として、以下に示すような効果を奏する。 As effects of the present invention, the following effects can be obtained.

即ち、本発明に係る浸窒焼入処理方法によれば、ワークに浸透拡散された窒素の浸窒濃度のバラつきをなくし、ひいては浸窒焼入処理品の品質を安定させることができる。 That is, according to the nitriding and quenching processing method according to the present invention, it is possible to eliminate variations in the nitriding concentration of nitrogen that has permeated and diffused into the workpiece, and to stabilize the quality of the nitriding and quenching processed product.

本発明に係る浸窒焼入処理方法の実施形態におけるフロー図。The flow figure in the embodiment of the nitriding quenching processing method concerning the present invention. 処理炉内を示す概略図。Schematic which shows the inside of a processing furnace. （ａ）は浸窒焼入処理を行う温度プロファイルを示す図、（ｂ）は浸窒焼入処理品の硬化層深さのバラつきを示す図。(A) is a figure which shows the temperature profile which performs a nitriding quenching process, (b) is a figure which shows the dispersion | variation in the hardening layer depth of a nitriding quenching processed product. （ａ）は窒素の浸入状態を示す模式図、（ｂ）は窒素の浸入状態を示す模式図。(A) is a schematic diagram which shows the infiltration state of nitrogen, (b) is a schematic diagram which shows the infiltration state of nitrogen.

次に、本発明に係る浸窒焼入処理方法の実施形態を、図１から図４を用いて説明する。
図１に示す如く、本発明の実施形態における浸窒焼入処理方法は、塑性流動層改良工程Ｓ１と、浸窒処理工程Ｓ２と、焼入処理工程Ｓ３と、を具備する。 Next, an embodiment of the nitriding quenching method according to the present invention will be described with reference to FIGS.
As shown in FIG. 1, the nitrous quenching method in the embodiment of the present invention includes a plastic fluidized bed improving step S1, a nitriding step S2, and a quenching step S3.

塑性流動層改良工程Ｓ１について説明する。塑性流動層改良工程Ｓ１とは、ワーク１の塑性流動層１０を改良する工程である。具体的には、図２に示す如く、まず、処理炉２内に前記ワーク１を設置する。
ここで、ワーク１とは、鉄鋼素材からなる浸窒焼入処理の対象物を示す。また、ワーク１の加工面の表層部分には、切断や切削等の加工抵抗による塑性流動層１０が存在している（図４（ａ）参照）。なお、本実施形態におけるワーク１は、その材質がＳＣＭ４２０鋼（炭素濃度：０．１８〜０．２３％）であるものとして説明する。
さらに、処理炉２とは、ワーク１の表面に窒素を浸透拡散させる装置であり、その内部には治具３が配置されている。ワーク１は、治具３上に載置されることによって、処理炉２内に設置される。また、処理炉２には、処理炉２内にアンモニアガス等のガスを供給するためのガス供給管４が接続されている。そして、ワーク１に浸窒処理を行うときには、ガス供給管４を通じて処理炉２内に所定流量のアンモニアガス等のガスが供給されるように構成されている。 The plastic fluidized bed improvement step S1 will be described. The plastic fluidized bed improvement step S1 is a step of improving the plastic fluidized bed 10 of the workpiece 1. Specifically, as shown in FIG. 2, first, the workpiece 1 is installed in the processing furnace 2.
Here, the work 1 indicates an object of nitrous quenching treatment made of a steel material. Moreover, the plastic fluidized layer 10 by processing resistance, such as a cutting | disconnection and cutting, exists in the surface layer part of the process surface of the workpiece | work 1 (refer Fig.4 (a)). In addition, the workpiece | work 1 in this embodiment demonstrates as what the material is SCM420 steel (carbon concentration: 0.18-0.23%).
Furthermore, the processing furnace 2 is a device for infiltrating and diffusing nitrogen on the surface of the workpiece 1, and a jig 3 is disposed inside the processing furnace 2. The workpiece 1 is placed in the processing furnace 2 by being placed on the jig 3. In addition, a gas supply pipe 4 for supplying a gas such as ammonia gas into the processing furnace 2 is connected to the processing furnace 2. When performing a nitriding process on the workpiece 1, a gas such as ammonia gas having a predetermined flow rate is supplied into the processing furnace 2 through the gas supply pipe 4.

次に、図３（ａ）に実線で示す如く、ワーク１が配置された処理炉２内の温度をワーク１のオーステナイト域温度、具体的には９００℃に昇温する。処理炉２内の温度は、図２に示す制御装置５によって制御される。
この場合、処理炉２内の温度が昇温すると、それに伴ってワーク１の温度も昇温し、このワーク１の温度は処理炉２内の温度と同等の温度にまで昇温する。 Next, as shown by a solid line in FIG. 3A, the temperature in the processing furnace 2 in which the workpiece 1 is disposed is raised to the austenite region temperature of the workpiece 1, specifically, 900 ° C. The temperature in the processing furnace 2 is controlled by the control device 5 shown in FIG.
In this case, when the temperature in the processing furnace 2 is increased, the temperature of the work 1 is increased accordingly, and the temperature of the work 1 is increased to a temperature equivalent to the temperature in the processing furnace 2.

ここで、オーステナイト域温度とは、所定の鉄鋼素材からなるワークを加熱していき、当該所定の鉄鋼素材からなるワークの結晶構造が、オーステナイト単層、または、オーステナイトとフェライトとの結晶構造に変態している状態の温度をいう。
本実施形態におけるワーク１のオーステナイト域温度は、ワーク１の炭素濃度が０．１８〜０．２３％であるため、ワーク１の結晶構造がフェライトとパーライトとの２層構造からオーステナイト単層の結晶構造へ変態している状態の温度域をいう。 Here, the austenite temperature means that a workpiece made of a predetermined steel material is heated and the crystal structure of the workpiece made of the predetermined steel material is transformed into an austenite single layer or a crystal structure of austenite and ferrite. It means the temperature in the state of being.
The austenite region temperature of the work 1 in this embodiment is that the carbon concentration of the work 1 is 0.18 to 0.23%, so the crystal structure of the work 1 is a crystal of an austenite single layer from a two-layer structure of ferrite and pearlite. The temperature range in which the structure is transformed.

さらに、図３（ａ）に実線で示す如く、昇温した処理炉２内の温度をそのまま保持することで、処理炉２内に配置されたワーク１の温度を、ワーク１のオーステナイト域温度に昇温した状態で所定時間保持する。具体的には、前記処理炉２内に配置されたワーク１の温度を９００℃に昇温し、係る状態を６０分間保持する。 Further, as shown by a solid line in FIG. 3A, the temperature of the workpiece 1 placed in the processing furnace 2 is changed to the austenite region temperature of the workpiece 1 by maintaining the heated temperature in the processing furnace 2 as it is. Hold the temperature for a predetermined time. Specifically, the temperature of the workpiece 1 arranged in the processing furnace 2 is raised to 900 ° C., and this state is maintained for 60 minutes.

ここで、図４（ａ）に示す如く、前記オーステナイト域温度に昇温される前段階におけるワーク１の加工面の表層部分には、塑性流動層１０が存在している。
しかしながら、上述のようにワーク１の温度を９００℃（オーステナイト域温度）に昇温することによって、ワーク１の転位は消滅し始める。さらに、当該９００℃の温度を６０分間保持することによって、図４（ｂ）に示す如く、ワーク１の転位が消滅してワーク１の表層部分における塑性流動層１０は消滅する。このように、処理炉２内に配置されたワーク１の温度を９００℃に昇温し、係る状態を６０分間保持することによって、ワーク１の塑性流動層１０は改良される。なお、ワーク１の温度をワーク１のオーステナイト域温度に昇温して保持する前記所定時間とは、ワーク１の温度をオーステナイト域温度に昇温した後、当該ワーク１の塑性流動層１０が消滅するまでの時間を示す。 Here, as shown in FIG. 4A, the plastic fluidized bed 10 exists in the surface layer portion of the processed surface of the workpiece 1 in the stage before the temperature is raised to the austenite region temperature.
However, the dislocation of the workpiece 1 starts to disappear by raising the temperature of the workpiece 1 to 900 ° C. (austenite temperature) as described above. Further, by holding the temperature of 900 ° C. for 60 minutes, the dislocation of the work 1 disappears and the plastic fluidized layer 10 in the surface layer portion of the work 1 disappears as shown in FIG. Thus, the plastic fluidized bed 10 of the workpiece 1 is improved by raising the temperature of the workpiece 1 placed in the processing furnace 2 to 900 ° C. and holding this state for 60 minutes. The predetermined time during which the temperature of the workpiece 1 is raised to the austenite region temperature of the workpiece 1 and maintained is the temperature of the workpiece 1 raised to the austenite region temperature, and then the plastic fluidized bed 10 of the workpiece 1 disappears. Shows the time until.

浸窒処理工程Ｓ２を説明する。浸窒処理工程Ｓ２とは、ワーク１のオーステナイト域温度に昇温されたワーク１の温度を浸窒処理温度に昇温または降温して、塑性流動層改良工程Ｓ１にて塑性流動層１０が改良された前記ワーク１に浸窒処理を行う工程である。
具体的には、図３（ａ）に実線で示す如く、処理炉２内の温度を８００℃に降温する。この場合、処理炉２内の温度が降温すると、それに伴ってワーク１の温度も降温し、このワーク１の温度は処理炉２内の温度と同等の温度にまで降温することとなる。そして、ガス供給管４を通じて当該処理炉２内に所定の流量のアンモニアガスおよび窒素ガスを供給し、係る状態で前記浸窒処理温度を１０５分間保持する。このようにしてワーク１に浸窒処理を行う。なお、浸窒処理温度とは、ワーク１に窒素を浸透拡散させるための温度をいう。 Nitrogen treatment process S2 is demonstrated. Nitrogenizing treatment step S2 is the temperature of workpiece 1 raised to the austenite region temperature of workpiece 1 is raised or lowered to the nitriding treatment temperature, and plastic fluidized bed 10 is improved in plastic fluidized bed improvement step S1. In this step, the workpiece 1 is subjected to a nitriding treatment.
Specifically, the temperature in the processing furnace 2 is lowered to 800 ° C. as indicated by a solid line in FIG. In this case, when the temperature in the processing furnace 2 is lowered, the temperature of the work 1 is also lowered accordingly, and the temperature of the work 1 is lowered to a temperature equivalent to the temperature in the processing furnace 2. Then, ammonia gas and nitrogen gas at a predetermined flow rate are supplied into the processing furnace 2 through the gas supply pipe 4, and the nitriding temperature is maintained for 105 minutes in such a state. In this way, the nitriding process is performed on the workpiece 1. The nitriding temperature refers to a temperature for infiltrating and diffusing nitrogen into the work 1.

焼入処理工程Ｓ３を説明する。焼入処理工程Ｓ３とは、浸窒処理が行われたワーク１に焼入処理を行う工程である。
具体的には、図３（ａ）に実線で示す如く、浸窒処理工程Ｓ２にて浸窒処理が行われたワーク１を、所定の油冷装置等で油急冷（例えば、６５℃で油急冷）することによって、ワーク１の結晶構造をマルテンサイトに変態させてワーク１を硬化させる。
このようにして、ワーク１の表面に硬化層が形成された浸窒焼入処理品を得る。 The quenching process S3 will be described. The quenching process S3 is a process of performing a quenching process on the workpiece 1 that has been subjected to the nitriding process.
Specifically, as shown by a solid line in FIG. 3A, the workpiece 1 that has been subjected to the nitriding treatment in the nitriding treatment step S2 is rapidly cooled with oil (for example, at 65 ° C. By rapidly cooling, the crystal structure of the work 1 is transformed into martensite and the work 1 is cured.
In this way, a nitriding and quenching treated product having a hardened layer formed on the surface of the workpiece 1 is obtained.

ここで、図３（ｂ）および図４（ａ）に示す如く、従来の浸窒焼入処理方法のように、表層部分に塑性流動層１０が存在するワーク１に対して浸窒焼入処理を行うと、当該ワーク１における硬化層深さ（浸窒深度）にバラつきが生じている。即ち、ワーク１に浸透拡散された窒素の浸窒濃度にバラつきが生じている。このことは、塑性流動層１０の粒界密度が大きい部分では窒素の浸透拡散が進行し易く、また、塑性流動層１０の粒界密度が小さい部分では窒素の浸透拡散が進行し難いために生じている。 Here, as shown in FIG. 3 (b) and FIG. 4 (a), as in the conventional nitriding and quenching method, the nitriding and quenching process is performed on the workpiece 1 in which the plastic fluidized bed 10 is present in the surface layer portion. As a result, the hardened layer depth (nitrogenation depth) in the workpiece 1 varies. That is, the nitrogen concentration of the nitrogen that has permeated and diffused into the work 1 varies. This is because the penetration and diffusion of nitrogen easily proceeds in the portion where the grain boundary density of the plastic fluidized bed 10 is large, and the penetration and diffusion of nitrogen hardly proceeds in the portion where the grain boundary density of the plastic fluidized bed 10 is small. ing.

しかしながら、本実施形態における浸窒焼入処理方法は、塑性流動層改良工程Ｓ１にてワーク１の塑性流動層１０を消滅させ、次いで、浸窒処理工程Ｓ２にて塑性流動層１０が消滅したワーク１に浸窒処理を行っている。このように塑性流動層１０が消滅したワーク１に浸窒処理を行うことで、図３（ｂ）および図４（ｂ）に示す如く、当該ワーク１における硬化層深さ（浸窒深度）、即ち、ワーク１に浸透拡散された窒素の浸窒濃度はバラつき無く安定することとなる。このことは、ワーク１の表層部分における粒界密度を均一なものとして、ワーク１への窒素の浸入拡散の容易さを一定にしたためである。
以上のことから、本発明に係る浸窒焼入処理方法の実施形態では、ワーク１に浸透拡散された窒素の浸窒濃度のバラつきを無くし、また、ワーク１に浸透拡散された窒素の浸窒濃度を任意の濃度にコントロールして、ひいては浸窒焼入処理品の品質を安定させることができる。 However, in the nitriding quenching method in the present embodiment, the plastic fluidized bed 10 of the workpiece 1 is extinguished in the plastic fluidized bed improvement step S1, and then the plastic fluidized bed 10 is extinguished in the nitrogenating treatment step S2. 1 is subjected to nitriding treatment. By performing nitriding treatment on the workpiece 1 in which the plastic fluidized bed 10 has disappeared in this way, as shown in FIGS. 3B and 4B, the depth of the hardened layer (nitrogenation depth) in the workpiece 1, In other words, the nitrogen concentration of nitrogen that has permeated and diffused into the workpiece 1 is stable without variation. This is because the grain boundary density in the surface layer portion of the work 1 is made uniform, and the ease of infiltration and diffusion of nitrogen into the work 1 is made constant.
From the above, in the embodiment of the nitriding and quenching treatment method according to the present invention, the variation in the nitriding concentration of nitrogen permeated and diffused into the work 1 is eliminated, and the nitrogen nitriding of the nitrogen permeated and diffused into the work 1 is eliminated By controlling the concentration to an arbitrary concentration, the quality of the nitrocarburized product can be stabilized.

また、ワーク１の温度をオーステナイト域温度に昇温させる手段として、高周波加熱装置を用いても良いものとする。このように、高周波加熱装置によりワーク１を昇温させることにより、本実施形態における浸窒焼入処理方法の処理時間を短縮することができ、また、ワーク１の必要な部位のみに対して当該浸窒焼入処理方法を施すことができる。 Further, a high-frequency heating device may be used as means for raising the temperature of the workpiece 1 to the austenite temperature. Thus, by raising the temperature of the workpiece 1 with the high-frequency heating device, the processing time of the nitriding quenching method in the present embodiment can be shortened, and only the necessary part of the workpiece 1 is concerned. Nitrogen quenching method can be applied.

１ワーク
１ａ塑性流動層
２処理炉
Ｓ１塑性流動層改良工程
Ｓ２浸窒処理工程
Ｓ３焼入処理工程 1 Workpiece 1a Plastic fluidized bed 2 Processing furnace S1 Plastic fluidized bed improvement process S2 Nitrogenation treatment process S3 Quenching process

Claims

鉄鋼素材からなるワークに窒素を浸透拡散させ、その後これに焼入れを行う、浸窒焼入処理方法であって、
該ワークの温度を該ワークのオーステナイト域温度に昇温して所定時間保持することによって、該ワークの塑性流動層を改良する、塑性流動層改良工程と、
前記ワークの温度を浸窒処理温度に昇温または降温して、塑性流動層が改良された前記ワークに浸窒処理を行う、浸窒処理工程と、
浸窒処理が行われた前記ワークに焼入処理を行う、焼入処理工程と、
を具備する、浸窒焼入処理方法。 Nitrogen quenching treatment method in which nitrogen is permeated and diffused into a workpiece made of steel material, and then quenched.
Improving the plastic fluidized bed of the workpiece by raising the temperature of the workpiece to the austenite region temperature of the workpiece and maintaining the workpiece for a predetermined time;
A nitriding treatment step of raising or lowering the temperature of the workpiece to a nitriding treatment temperature and performing a nitriding treatment on the workpiece whose plastic fluidized bed is improved;
A quenching process for performing a quenching process on the workpiece subjected to the nitriding process,
A nitriding quenching method comprising: