Gas nitrocarburizing method and method for manufacturing bearing part

• US 10,047,429 B2
• Filed: 04/09/2012
• Issued: 08/14/2018
• Est. Priority Date: 04/19/2011
• Status: Active Grant

• Current Assignee: NTN Corporation
• Original Assignee: NTN Corporation
• Inventors: Ohki, Chikara
• Primary Examiner(s): Roe, Jessee R
• Assistant Examiner(s): Koshy, Jophy S.
• Application Number:
  US14/112,871
• Publication Number:
  US 20140041763A1
• Time in Patent Office: 2,318 Days
• CPC Class Codes: C21D 1/06 Surface hardening
• CPC Class Codes: C21D 1/74 Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
• CPC Class Codes: C21D 1/76 Adjusting the composition of the atmosphere
• CPC Class Codes: C21D 9/36 for balls; for rollers
• CPC Class Codes: C21D 9/38 for roll bodies
• CPC Class Codes: C21D 9/40 for rings; for bearing races
• CPC Class Codes: C23C 8/26 of ferrous surfaces
• CPC Class Codes: C23C 8/32 of ferrous surfaces

Abstract

Provided is a gas nitrocarburizing method forming a nitride layer in a surface layer portion of a workpiece made of steel by heating the workpiece within a heat treatment furnace into which a heat treatment gas is introduced, the heat treatment gas containing ammonia gas and at least one of carbon dioxide gas and hydrogen gas, and having a remainder formed of an impurity.

Claims

1. A gas nitrocarburizing method forming a nitride layer in a surface layer portion of a workpiece made of steel by heating said workpiece within a heat treatment furnace into which a heat treatment gas is introduced,

said heat treatment gas containing ammonia gas, hydrogen gas and carbon dioxide gas,

wherein said nitride layer is formed by heating said workpiece to a temperature range of more than or equal to 550°

C. and less than or equal to 650°

C. within said heat treatment furnace; and

wherein an undecomposed ammonia fraction in an atmosphere within said heat treatment furnace is controlled such that a difference between a maximum value and a minimum value of the undecomposed ammonia fraction in said atmosphere obtained at a plurality of positions within said heat treatment furnace is less than or equal to 0.8% by increasing a flow rate of the carbon dioxide gas and a flow rate of the hydrogen gas,

wherein a ratio of said flow rate of said carbon dioxide gas to a total flow rate of said heat treatment gas introduced into said heat treatment furnace is more than or equal to 5% and less than or equal to 20%, and

wherein a ratio of said flow rate of said hydrogen gas to the total flow rate of said heat treatment gas introduced into said heat treatment furnace is more than or equal to 10% and less than or equal to 50%.

2. The gas nitrocarburizing method according to claim 1, wherein said workpiece is heated within said heat treatment furnace with an atmosphere within said heat treatment furnace being stirred by a stirring fan arranged within said heat treatment furnace.