U.S. patent application number 10/519711 was filed with the patent office on 2006-05-18 for surface modified stainless steel.
This patent application is currently assigned to SANDVIK INTELLECTUAL PROPERTY AB. Invention is credited to Goran Berglund.
Application Number | 20060102253 10/519711 |
Document ID | / |
Family ID | 20288446 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060102253 |
Kind Code |
A1 |
Berglund; Goran |
May 18, 2006 |
Surface modified stainless steel
Abstract
A stainless steel, which after nitriding exhibits a hardened
surface layer with a hardness of at least 1200 Hv is disclosed. The
stainless steel can be in the form of wire, plate, strip, tube and
pipe and other geometries, especially complex geometries,
particularly useful in applications with high demands on a
combination of high strength and/or toughness and wear resistance
and as a substrate for coating.
Inventors: |
Berglund; Goran; (Sandviken,
SE) |
Correspondence
Address: |
DRINKER BIDDLE & REATH (DC)
1500 K STREET, N.W.
SUITE 1100
WASHINGTON
DC
20005-1209
US
|
Assignee: |
SANDVIK INTELLECTUAL PROPERTY
AB
Sandviken
SE
|
Family ID: |
20288446 |
Appl. No.: |
10/519711 |
Filed: |
July 2, 2003 |
PCT Filed: |
July 2, 2003 |
PCT NO: |
PCT/SE03/01159 |
371 Date: |
December 12, 2005 |
Current U.S.
Class: |
148/222 ;
148/230; 148/318; 420/38 |
Current CPC
Class: |
C22C 38/50 20130101;
C22C 38/44 20130101; C23C 8/26 20130101; C22C 38/42 20130101; C22C
38/06 20130101 |
Class at
Publication: |
148/222 ;
148/230; 148/318; 420/038 |
International
Class: |
C22C 38/52 20060101
C22C038/52 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 3, 2002 |
SE |
02022107-9 |
Claims
1. A stainless steel comprising a composition (in weight-%):
TABLE-US-00004 Carbon max 0.1 Nitrogen max 0.1 Copper 0.5-4
Chromium 10-14 Molybdenum 0.5-6 Nickel 7-11 Cobalt 0-9 Tantalum max
0.1 Niobium max 0.1 Vanadium max 0.1 Tungsten max 0.1 Aluminum
0.05-0.6 Titanium 0.4-1.4 Silicon max 0.7 Manganese .ltoreq.1.0
Iron balance and normally occurring usual steelmaking additions and
impurities, wherein said stainless steel after nitriding exhibits a
hardened surface layer with a hardness of at least 1200 Hv.
2-5. (canceled)
6. The stainless steel according to claim 1, wherein the stainless
steel includes quasicrystalline particles in a martensitic
microstructure.
7. The stainless steel according to claim 6, wherein the
quasicrystalline particles in the martensitic microstructure are a
result of a precipitation hardening process.
8. The stainless steel according to claim 1, wherein a hardness at
a surface of the stainless steel is at least twice that of a
hardness of at 0.5 mm into a matrix of the stainless steel.
9. The stainless steel according to claim 1, wherein the hardened
surface layer has a thickness of about 0.5 mm.
10. The stainless steel according to claim 1, wherein the stainless
steel is formed into one or more of a wire, a plate, a strip, tube
and a pipe.
11. The stainless steel according to claim 1, wherein the stainless
steel is formed into a complex geometry for use in an application
with a high demand on a combination of high strength and/or
toughness and wear resistance.
12. The stainless steel according to claim 11, wherein the complex
geometry is a wear part of an engine, an engine component, or an
impact load.
13. The stainless steel according to claim 11, wherein the complex
geometry is a cam follower, a cam follower pad, a valve stem, a
valve stem guide, a piston pin, a piston shaft, a hydraulic piston,
an ejector pin, a safety protection plate, a lock cylinder and
other locking devices, a blocking element, or a thief-proof
equipment
14. A material comprising a wear resistant coating deposited on the
stainless steel according to claim 1.
15. A method for making a surface modified stainless steel, the
method comprising: subjecting a stainless steel to a nitriding
process at a temperature of 450 to 580.degree. C. for a time period
of 1 to 40 hours in a plasma nitriding atmosphere, the stainless
steel having a composition comprising: TABLE-US-00005 Carbon max
0.1 Nitrogen max 0.1 Copper 0.5 to 4 Chromium 10 to 14 Molybdenum
0.5 to 6 Nickel 7 to 11 Cobalt 0 to 9 Tantalum max 0.1 Niobium max
0.1 Vanadium max 0.1 Tungsten max 0.1 Aluminum 0.05 to 0.6 Titanium
0.4 to 1.4 Silicon max 0.7 Manganese .ltoreq.1.0
Iron balance and normally occurring usual steelmaking additions and
impurities.
16. The method according to claim 15, wherein said stainless steel
after nitriding exhibits a hardened surface layer with a hardness
of at least 1200 Hv.
17. The method according to claim 15, wherein the surface modified
stainless steel does not change dimension from the nitriding
process.
Description
RELATED APPLICATION DATA
[0001] This application is a .sctn.371 National Stage Application
of PCT International Application No. PCT/SE2003/001159 filed Jul.
2, 2003, which International Application was published by the
International Bureau in English on Jan. 15, 2004, the entire
contents of which are incorporated herein by reference. This
application also claims priority under 35 U.S.C. .sctn.119 and/or
.sctn.365 to Swedish Application No. 0202107-9, filed Jul. 3, 2002,
the entire contents of which are incorporated herein by
reference.
FIELD OF THE DISCLOSURE
[0002] The present disclosure relates to a stainless steel, which
after nitriding exhibits a hardened surface layer with a hardness
of at least 1200 Hv. Such stainless steel is particularly useful,
for example, in applications with high demands on a combination of
high strength and/or toughness and wear resistance and as a
substrate for coating.
STATE OF THE ART
[0003] In the discussion of the state of the art that follows,
reference is made to certain structures and/or methods. However,
the following references should not be construed as an admission
that these structures and/or methods constitute prior art.
Applicant expressly reserves the right to demonstrate that such
structures and/or methods do not qualify as prior art against the
present invention.
[0004] Stainless steel alloys are relatively less hard than other
steel materials. As a result, in many applications the stainless
steel article or part is provided with a hardened surface, often
referred to as case hardening. The concept of case hardening is to
transform a relatively thin layer of material at the surface of the
part by enrichment of carbon or other ingredients to make the
surface harder than the matrix of the alloy. The steel thus retains
in bulk the desired formality of stainless steel without the
softness of the matrix at the modified steel surface.
[0005] Stainless steels are often casehardened by carburization.
Carburization is a process by which carbon atoms are diffused in
solution into the surface of the article. Known case hardening
processes are performed at high temperatures. However,
carburization processes performed at temperatures greater than
about 540.degree. C. (for stainless steel alloys) can promote the
formation of carbides in the hardened surface.
[0006] Plasma nitriding is an alternative case-hardening process.
Plasma nitriding is carried out in a glow discharge in a nitrogen
gas-containing mixture at a pressure of 100 to 1000 Pa (1 to 10
mbar). This method treats stainless steel surfaces, resulting in a
nitrogen diffusion layer having high hardness and excellent wear
resistance. Nitriding hardening is induced by the precipitation of
nitrides in the surface layer.
[0007] Plasma nitriding is a recently developed surface hardening
procedure. The process can replace traditional nitriding methods,
such as gas nitriding and nitrocarburation (short-term gas
nitriding, bath nitriding and tenifer treatment) as similar
thermo-chemical conditions can be established in this process.
Plasma nitriding achieves higher hardness and wear resistance,
while creating lower distortion.
[0008] Plasma nitriding is very cost effective. This is due to the
fact that subsequent machining, finishing and residue-removal
processes are frequently not required. Similarly, supplementary
protective measures, such as burnishing, phosphatizing, etc., under
some conditions even galvanizing and hard-chrome plating, may not
be necessary.
[0009] Plasma nitriding is performed in a vacuum furnace. Treatment
temperatures in the range of 400 to 580.degree. C. are employed
subject to the requirements of the process at hand. Typical
treatment temperatures are in the range of 420 to 500.degree. C.
Commonly used process gases are ammonia, nitrogen, methane, and
hydrogen. Oxygen and carbon dioxide are used in the corrosion
protective step of post-oxidation. Aside from the type of process
gas used, pressure, temperature, and time are the main parameters
of the treatment process. By varying these parameters, the plasma
nitriding process can be fine tuned to achieve the exact desired
properties in any treated component.
[0010] Any iron-based material can be subjected to plasma
nitriding. The process does not require the use of special types of
nitriding steel. The results achieved through plasma nitriding can
be accurately reproduced. This is especially important in the
manufacture of serial products. U.S. Pat. No. 5,632,826 discloses a
precipitation hardened martensitic alloy in which the strengthening
is based on the precipitation of particles. The strengthening
particles have a quasicrystalline structure, said structure being
essentially obtained at aging times up to 1000 h and tempering
treatments up to 650.degree. C. This strengthening involves an
increase in tensile strength of at least 200 MPa. It has now
surprisingly been found that if steel according to U.S. Pat. No.
5,632,826 is nitrided on the surface, an unexpected further
increase in surface hardness is obtained compared to the matrix of
said stainless steel.
SUMMARY
[0011] An exemplary embodiment of a stainless steel comprises a
composition (in weight-%): TABLE-US-00001 Carbon max 0.1 Nitrogen
max 0.1 Copper 0.5 to 4 Chromium 10 to 14 Molybdenum 0.5 to 6
Nickel 7 to 11 Cobalt 0 to 9 Tantalum max 0.1 Niobium max 0.1
Vanadium max 0.1 Tungsten max 0.1 Aluminum 0.05 to 0.6 Titanium 0.4
to 1.4 Silicon max 0.7 Manganese .ltoreq.1.0
[0012] Iron balance and normally occurring usual steelmaking
additions and impurities, wherein said stainless steel after
nitriding exhibits a hardened surface layer with a hardness of at
least 1200 Hv.
[0013] An exemplary method for making a surface modified stainless
steel comprises subjecting a stainless steel to a nitriding process
at a temperature of 450 to 580.degree. C. for a time period of 1 to
40 hours in a plasma nitriding atmosphere, the stainless steel
having a composition comprising: TABLE-US-00002 Carbon max 0.1
Nitrogen max 0.1 Copper 0.5 to 4 Chromium 10 to 14 Molybdenum 0.5
to 6 Nickel 7 to 11 Cobalt 0 to 9 Tantalum max 0.1 Niobium max 0.1
Vanadium max 0.1 Tungsten max 0.1 Aluminum 0.05 to 0.6 Titanium 0.4
to 1.4 Silicon max 0.7 Manganese .ltoreq.1.0
[0014] Iron balance and normally occurring usual steelmaking
additions and impurities.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The following detailed description of preferred embodiments
can be read in connection with the accompanying drawings in which
like numerals designate like elements and in which:
[0016] FIG. 1 is a light-optical micrograph showing the
microstructure of an exemplary embodiment of the surface modified
stainless steel in 500.times., where A is the nitrided surface
layer and B is the stainless steel matrix.
[0017] FIG. 2 is a graph showing the hardness (in Hv) plotted over
the depth (in mm) from the surface.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0018] A stainless steel alloy characterized by increased hardness
at the surface of said alloy after modification of the surface at
the same time as the hardness of the matrix of the stainless steel
is also increased is provided. In addition, products made of said
surface modified stainless steel are provided. Further, a stainless
steel substrate for coating with wear resistant layers is
provided.
[0019] In exemplary embodiments, the stainless steel substrate
before surface modification has the following composition (in
weight-%): TABLE-US-00003 Carbon max 0.1 Nitrogen max 0.1 Copper
0.5 to 4 Chromium 10 to 14 Molybdenum 0.5 to 6 Nickel 7 to 11
Cobalt 0 to 9 Tantalum max 0.1 Niobium max 0.1 Vanadium max 0.1
Tungsten max 0.1 Aluminum 0.05 to 0.6 Titanium 0.4 to 1.4 Silicon
max 0.7 Manganese .ltoreq.1.0
[0020] Iron balance and normally occurring usual steelmaking
additions and impurities.
[0021] Said stainless steel contains quasicrystalline particles in
the martensitic microstructure as a result of a precipitation
hardening.
[0022] Plasma nitriding is a surface hardening process, which
utilizes the properties of gas plasma, i.e., an ionized gas, to
achieve desirable mechanical properties at the surface of the work
piece. The main influential parameters in nitriding are pressure,
temperature, and time of treatment as well as the chemical
composition of the ionized process gas. Plasma nitriding typically
takes place at a vacuum pressure between 0.3 to 10 mbar. The actual
treatment pressure chosen is governed by the geometry of the part
and the desired surface layer structure.
[0023] The treatment temperature in the range of 400 to 580.degree.
C. is selected according to the type of material and pre-treatment
of the part and the desired layer structure. Treatment time varies
between 10 minutes and 70 hours, and depends on the part to be
treated as well as the desired structure and thickness of the
layers formed. Plasma nitriding uses ammonia or gas mixtures
containing methane, nitrogen, and hydrogen as the process gas. The
process gas used is selected subject to the nature of the part to
be treated and the required layer structure.
[0024] In exemplary embodiments, a material treated with the
disclosed method can be in the form of wire, plate, strip, tube and
pipe and other geometries, especially complex geometries for use in
applications with high demands on a combination of high strength
and/or toughness and wear resistance, such as, e.g., wear parts of
engines and other engine components, impact loads, such as safety
devices, cam followers, cam follower pads, valve stems, valve stem
guides, piston pins, piston shafts, hydraulic pistons, ejector
pins, safety protection plates, lock cylinders and other locking
devices, blocking elements, thief-proof equipment or the like.
EXAMPLE 1
[0025] A stainless steel substrate as described herein was
subjected to a surface modification by a plasma nitriding process
at 450 to 580.degree. C. during a period of time of 1 to 40 hours.
This process obtains a hardening of the surface between 0.05 and
0.5 mm. The hardening process can be carried out on wire, plate,
strip, tube and pipe and parts with a wide variation of geometries,
especially complex geometries. It is a special advantage of the
stainless steel substrate used according to the disclosed process,
that very complex geometries can be formed without any changes in
dimension. The hardness of the surface is at least twice the
hardness of the substrate 0.5 mm into the matrix. The hardness of
the surface is 1200 Hv, alternatively at least 1100 Hv.
[0026] FIG. 2 is a graph illustrating the hardness profile from the
surface of the substrate into the matrix. It unexpectedly shows
that the hardening effect is visible down to about 0.5 mm into the
matrix. It is therefore considered a big advantage of this
combination of substrate and the method of surface treatment, that
creates a surface modified material with a deep-hardened surface
zone.
[0027] Exemplary embodiments of the surface modified stainless
steel according to the present invention is particularly well
suited for use as substrate for the deposition of a wear resistant
coating.
[0028] Although the present invention has been described in
connection with preferred embodiments thereof, it will be
appreciated by those skilled in the art that additions, deletions,
modifications, and substitutions not specifically described may be
made without department from the spirit and scope of the invention
as defined in the appended claims.
* * * * *