U.S. patent application number 10/755347 was filed with the patent office on 2004-09-09 for surface modified precipitation hardened stainless steel.
This patent application is currently assigned to Sandvik Aktiebolag. Invention is credited to Berglund, Goran.
Application Number | 20040173288 10/755347 |
Document ID | / |
Family ID | 20290113 |
Filed Date | 2004-09-09 |
United States Patent
Application |
20040173288 |
Kind Code |
A1 |
Berglund, Goran |
September 9, 2004 |
Surface modified precipitation hardened stainless steel
Abstract
The present invention relates to a surface hardened and coated
stainless steel said surface showing low static friction and with
improved wear resistance. Moreover, it relates to a coating of the
surface of said stainless steel, in which a surface hardening is
accomplished simultaneously with said coating. The resulting coated
steel showing a very high hardness simultaneously as it shows
improved adherence. Said steel advantageously being used in
applications with high requirements regarding a combination of high
strength and/or toughness and wear resistance together with low
friction, such as, e.g., shock absorbers and items for combustion
engines and hydraulic systems, produced with a highly cost
effective process. The used precipitation hardened stainless steel
substrate has the following composition (in weight-%): 1 Carbon max
about 0.1 Nitrogen max about 0.1 Copper from about 0.5 to about 4
Chromium from about 10 to about 14 Molybdenum from about 0.5 to
about 6 Nickel from about 7 to about 11 Cobalt 0 up to about 9
Tantalum max about 0.1 Niobium max about 0.1 Vanadium max about 0.1
Tungsten max about 0.1 Aluminum from about 0.05 to about 0.6
Titanium from about 0.4 to about 1.4 Silicon max about 0.7
Manganese max about 1.0 Iron balance and normally occurring usual
steelmaking additions and impurities.
Inventors: |
Berglund, Goran; (Sandviken,
SE) |
Correspondence
Address: |
BURNS DOANE SWECKER & MATHIS L L P
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
Sandvik Aktiebolag
Sandviken
SE
|
Family ID: |
20290113 |
Appl. No.: |
10/755347 |
Filed: |
January 13, 2004 |
Current U.S.
Class: |
148/220 ;
148/222; 148/326 |
Current CPC
Class: |
C21D 6/004 20130101;
C21D 6/02 20130101; F16F 2226/02 20130101; C23C 30/00 20130101;
C21D 1/06 20130101 |
Class at
Publication: |
148/220 ;
148/222; 148/326 |
International
Class: |
C23C 008/38 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 13, 2003 |
SE |
0300073-4 |
Claims
1. A coated, surface hardened precipitation hardened stainless
steel with following composition (in weight-%):
8 Carbon max about 0.1 Nitrogen max about 0.1 Copper from about 0.5
to about 4 Chromium from about 10 to about 14 Molybdenum from about
0.5 to about 6 Nickel from about 7 to about 11 Cobalt 0 up to about
9 Tantalum max about 0.1 Niobium max about 0.1 Vanadium max about
0.1 Tungsten max about 0.1 Aluminum from about 0.05 to about 0.6
Titanium from about 0.4 to about 1.4 Silicon max about 0.7
Manganese max about 1.0 Iron balance
and normally occurring usual steelmaking additions and impurities
wherein said steel is coated and surface hardened in one and the
same operation, having a combination of high strength and/or
toughness and wear resistance together with low friction and
improved adhesiveness.
2. The precipitation hardened stainless steel of claim 1 wherein
said coating is applied on a nitrided surface of said steel.
3. The precipitation hardened stainless steel of claim 1 wherein
said coating consists essentially of a single layer.
4. The precipitation hardened stainless steel according to claim 1
wherein said coating consists essentially of a single layer of
diamond-like carbon (DLC).
5. The precipitation hardened stainless steel of claim 1 wherein
said coating consists essentially of a single layer of diamond-like
carbon (DLC) with addition of tungsten carbide.
6. The precipitation hardened stainless steel of claim 1 wherein
said coating consists essentially of titanium nitride.
7. The precipitation hardened stainless steel of claim 1 wherein
said precipitation hardened stainless steel has been strengthened
by the precipitation of quasi crystalline structured particles.
8. A process for producing a stainless steel with a low static
friction on a very hard and wear resistant surface, said process
comprising using PVD to apply a low static friction coating in the
same operation as surface hardening, said stainless steel having
the following composition (in weight-%):
9 Carbon max about 0.1 Nitrogen max about 0.1 Copper from about 0.5
to about 4 Chromium from about 10 to about 14 Molybdenum from about
0.5 to about 6 Nickel from about 7 to about 11 Cobalt 0 up to about
9 Tantalum max about 0.1 Niobium max about 0.1 Vanadium max about
0.1 Tungsten max about 0.1 Aluminum from about 0.05 to about 0.6
Titanium from about 0.4 to about 1.4 Silicon max about 0.7
Manganese max about 1.0 Iron balance
and normally occurring usual steelmaking additions and
impurities.
9. The process of claim 7 wherein the low static friction coating
comprises one or more of diamond-like carbon (DLC), diamond-like
carbon with addition of titanium nitride.
10. A process for producing a stainless steel with a low static
friction on a very hard and wear resistant surface, said process
comprising using PVD to apply a low static friction coating onto a
plasma-nitrided surface of the stainless steel in the same
operation, said stainless steel having the following composition
(in weight-%):
10 Carbon max about 0.1 Nitrogen max about 0.1 Copper from about
0.5 to about 4 Chromium from about 10 to about 14 Molybdenum from
about 0.5 to about 6 Nickel from about 7 to about 11 Cobalt 0 up to
about 9 Tantalum max about 0.1 Niobium max about 0.1 Vanadium max
about 0.1 Tungsten max about 0.1 Aluminum from about 0.05 to about
0.6 Titanium from about 0.4 to about 1.4 Silicon max about 0.7
Manganese max about 1.0 Iron balance
and normally occurring usual steelmaking additions and
impurities.
11. The process of claim 9 wherein the low static friction coating
comprises one or more of diamond-like carbon (DLC), diamond-like
carbon with addition of titanium nitride.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a surface hardened and
coated precipitation hardened stainless steel said surface showing
low static friction and improved wear resistance. Moreover, it
relates to a coating of the surface of said stainless steel, in
which a surface hardening is accomplished simultaneously with said
coating. The resulting coated steel showing a very high hardness
simultaneously as it shows improved adhesitivity. This steel can
advantageously be used in applications with high requirements
regarding a combination of high strength and/or toughness and wear
resistance together with low friction, such as, e.g., shock
absorbers and items for combustion engines and hydraulic systems,
produced with a highly cost effective process.
BACKGROUND OF THE INVENTION
[0002] Normally, stainless steel alloys are softer than other steel
materials. Therefore, they are frequently submitted to a hardening
treatment, which basically may be a bulk treatment or a surface
treatment. The bulk treatment is intended to harden the steel
material homogeneously, such as a plate or a wire, throughout the
entire cross-section of the material, while the surface treatment
is intended to harden only the surface of the component, leaving
the substrate substantially unaffected.
[0003] For instance, U.S. Pat. No. 5,632,826 (&WO-A-95/09930),
which is hereby included in its entirety into the disclosure of the
present application by this reference, discloses a precipitation
hardened stainless steel in which the strengthening is based on the
precipitation of particles throughout the material. The
strengthening particles have a quasi-crystalline structure, said
structure being essentially obtained at aging times up to about
1000 hours and tempering treatments up to about 650.degree. C. This
strengthening involves an increase in tensile strength of at least
200 MPa.
[0004] Other processes for precipitation hardening stainless steel
and/or components made of said steel are disclosed in
WO-A-93/07303, WO-A-01/36699 and WO-A-01/14601, which hereby are
all incorporated into the disclosure of the present application by
this reference. For example, according to WO-A-01/36699, the
production of the material prior to aging/hardening shall be such
that the item be subjected to cold forming to a degree of
deformation sufficient for obtaining a martensite content of at
least 50% preferably at least 70%.
[0005] Instead of a hardening treatment affecting the steel
throughout and homogeneously, in many applications the stainless
steel component is provided with a hardened surface, often referred
to as casehardening. The concept of casehardening is to transform a
relatively thin layer of material at the surface of the part by
enrichment of carbon or other ingredients, in order to make the
surface harder than the substrate, the substrate being the bulk of
the steel that remains unaffected by the surface modification.
[0006] Stainless steels are often casehardened by carburization.
This is a process where carbon atoms in solution diffuse into the
surface of the substrate, i.e., the steel. Known casehardening
processes are performed at high temperatures. Carburization
processes are performed at temperatures of about 540.degree. C. or
higher (for stainless steel alloys). However, such high temperature
processes can promote the formation of carbides in the surface of
said stainless steel.
[0007] In many mechanical applications, not only the hardness but
also the static friction, as indicated above, of the steel surface
is a known problem. Even if lubrication is made, the static
friction may cause considerable friction loss, especially in cases
where a reciprocal movement will be performed. Examples of such
applications are, e.g., shock absorbers for vehicles, hydraulic
systems in the process industry and internal items of combustion
engines, such as cam followers. At high-frequency changes of the
type of movement or its direction, the static friction may cause a
local temperature increase, which leads to deteriorated performance
and/or service time and risks of leakage.
[0008] In order to decrease the static friction, exposed surfaces
are usually coated with some form of layer with better properties
than the under-lying steel substrate. Besides giving a lower
friction, one desired property of said layer is to protect the
substrate against mechanical wear. Therefore, the applied layer
should be as hard as possible. In hydraulic steering control
equipment in the process industry, a high static friction may cause
a movement resistance, which deteriorates the precession of the
hydraulic component. Said problems with static friction can occur,
e.g., in combustion engines, in a cam follower for inlet and outlet
valves. The surface on which the follower acts is exposed to a very
high local load, which may result in serious wear problems.
[0009] A conventional way of lowering the static friction and to
increase the hardness is to prepare a very smooth surface and then
to apply hard chromium plating on this surface. Thereby a hardness
level is achieved for low alloy wrought steel that amounts to about
1000 Hv. In order to support the layer, a surface hardening is
often made before the hard chromium plating. The process is
relatively complicated and involves several relocations of the
work-piece due to the dimension alterations it undergoes during the
hardening.
[0010] One possible solution of the problem of the difference in
hardness of the substrate and the applied coating is to apply a
system of layers. A treated work-piece comprises, e.g., of a base
body or substrate of steel and a hard material layer system next to
the substrate, supplemented by a metal layer and finally a sliding
layer system, whereby the latter is preferably made of carbide,
especially tungsten carbide or chromium carbide, and dispersed
carbon. Although good hardness values and low static friction are
achieved, the composite system of several layers is complicated,
time-consuming and expensive to produce.
[0011] Another alternative is build a layer system, which comprises
of an adhesive layer, which is placed on a substrate, a transition
layer, which is placed on the adhesive layer and an outer layer,
which is made of diamond-like carbon. The adhesive layer comprises
at least one element from the group consisting, e.g., of the
4.sup.th, 5.sup.th and 6.sup.th subgroups and silicon. The
transition layer consists of diamond-like carbon. The layer system
has a hardness of at least 15 GPa, preferably at least 20 GPa, and
an adhesive strength of at least 3 HF according to VDI 3824
("Quality Assurance in the Case of PVD and CVD Hard Coatings"),
sheet 4.
OBJECTS AND SUMMARY OF THE INVENTION
[0012] In view of the above, it is a primary object of the present
invention to obtain a low static friction and wear resistant
stainless steel surface.
[0013] Another object of the present invention is to obtain a low
static friction on a very hard and wear resistant stainless steel
surface in a simple and cost effective way, with as few procedural
steps as possible.
[0014] Still another object of the present invention to produce
components of sophisticated geometry of said stainless steel with a
low static friction on a very hard and wear resistant surface.
[0015] In one aspect there is provided a coated, surface hardened
precipitation hardened stainless steel with following composition
(in weight-%):
2 Carbon max about 0.1 Nitrogen max about 0.1 Copper from about 0.5
to about 4 Chromium from about 10 to about 14 Molybdenum from about
0.5 to about 6 Nickel from about 7 to about 11 Cobalt 0 up to about
9 Tantalum max about 0.1 Niobium max about 0.1 Vanadium max about
0.1 Tungsten max about 0.1 Aluminum from about 0.05 to about 0.6
Titanium from about 0.4 to about 1.4 Silicon max about 0.7
Manganese max about 1.0 Iron balance
[0016] and normally occurring usual steelmaking additions and
impurities wherein said steel is coated and surface hardened in one
and the same operation, having a combination of high strength
and/or toughness and wear resistance together with low friction and
improved adhesiveness.
[0017] In another aspect there is provided a process for producing
a stainless steel with a low static friction on a very hard and
wear resistant surface, said process comprising using a PVD to
apply a low static friction coating in the same operation as
surface hardening, said stainless steel having the following
composition (in weight-%):
3 Carbon max about 0.1 Nitrogen max about 0.1 Copper from about 0.5
to about 4 Chromium from about 10 to about 14 Molybdenum from about
0.5 to about 6 Nickel from about 7 to about 11 Cobalt 0 up to about
9 Tantalum max about 0.1 Niobium max about 0.1 Vanadium max about
0.1 Tungsten max about 0.1 Aluminum from about 0.05 to about 0.6
Titanium from about 0.4 to about 1.4 Silicon max about 0.7
Manganese max about 1.0 Iron balance
[0018] and normally occurring usual steelmaking additions and
impurities.
[0019] In a further aspect providing a process for producing a
stainless steel with a low static friction on a very hard and wear
resistant surface, said process comprising using PVD to apply a low
static friction coating onto a plasma-nitrided surface of the
stainless steel in the same operation, said stainless steel having
the following composition (in weight-%):
4 Carbon max about 0.1 Nitrogen max about 0.1 Copper from about 0.5
to about 4 Chromium from about 10 to about 14 Molybdenum from about
0.5 to about 6 Nickel from about 7 to about 11 Cobalt 0 up to about
9 Tantalum max about 0.1 Niobium max about 0.1 Vanadium max about
0.1 Tungsten max about 0.1 Aluminum from about 0.05 to about 0.6
Titanium from about 0.4 to about 1.4 Silicon max about 0.7
Manganese max about 1.0 Iron balance
[0020] and normally occurring usual steelmaking additions and
impurities.
DETAILED DESCRIPTION OF THE INVENTION
[0021] Thus, the present invention relates to methods of
application of a low static friction coating on a specific class of
stainless steels. Moreover, this low static friction coating also
results in a very hard and wear resistant surface. The coating is
applied according to the well-known PVD ("Physical Vapor
Deposition") technique, in accordance with the state of the art
referred to above. The steel has turned out to possess the
surprising property of having a considerable inner hardness
increase when the coating is applied whereby the necessary hard and
carrying surface layer is created to carry the hard and
low-friction top coating. Since the PVD operation is performed at a
relatively low temperature, the dimensions of the work-piece are
maintained without any distortions. The utilization of the PVD
technique on some special stainless steel alloys brings about a
number of advantages for the produciton of, e.g., cylinder tubes
and piston rods for shock absorbers, pistons for hydraulic guide
means, and cam followers for combustion engines.
[0022] Before any surface modifications, a suitable group of
stainless steels for the purposes of the present invention was
selected. It has the following composition ranges (in weight
%):
5 Carbon max about 0.1 Nitrogen max about 0.1 Copper from about 0.5
to about 4 Chromium from about 10 to about 14 Molybdenum from about
0.5 to about 6 Nickel from about 7 to about 11 Cobalt 0 up to about
9 Tantalum max about 0.1 Niobium max about 0.1 Vanadium max about
0.1 Tungsten max about 0.1 Aluminum from about 0.05 to about 0.6
Titanium from about 0.4 to about 1.4 Silicon max about 0.7
Manganese max about 0.1 Iron balance
[0023] and normally occurring usual steelmaking additions and
impurities.
[0024] This stainless steel contains quasi-crystalline particles in
a martensitic microstructure as a result of a precipitation
hardening, as described in the above mentioned prior art references
U.S. Pat. No. 5,632,826, WO-A-93/07303, WO-A-01/14601 and
WO-A-01/36699.
[0025] In order to bring about a surface treatment according to the
present invention, a specific precipitation hardened stainless
steel was chosen having the following composition (in weight
%):
6 C + N max about 0.05 Cr 12.00 Mn 0.30 Ni 9.00 Mo 4.00 Ti 0.90 Al
0.30 Si 0.15 Cu 2.00 Fe Balance
[0026] On this steel, a low static friction coating is applied,
said coating consisting essentially of titanium nitride or
diamond-like carbon (DLC), which is applied by PVD technique. This
includes the metal piece being exposed to a temperature between
about 450 and about 500.degree. C. for a couple of hours. In the
same temperature region, and after determined intervals, a
hardening of the steel takes place, whereby hardness in the
magnitude of 650 Hv is attained. In this way, an excellent support
for the coating is obtained in the same operation. Thanks to the
relatively low treatment temperature, the work-piece maintains its
shape and dimensions with high accuracy, which results in a
considerably simplified working process. At the same time, in spite
of a thinner layer, the thickness of which is in the order of 6
.mu.m, a superior wear resistance is obtained in comparison to
conventional 25 .mu.m thick hard chromium layers on a hardened
surface. Thus, the great advantage of the present invention is that
the application of the low static friction and wear resistant
coating and the necessary surface hardening are brought about in
one and the same operation.
[0027] Another significant advantage of the present invention is
when the work-piece is of tubular shape for the manufacturing of
tube-shaped items. Thanks to an excellent cold-workability of the
stainless steel according to the invention, tubular products are
readily produced. Costly long-hole drilling operations otherwise
required for commonly available bar shaped products are thus
eliminated.
[0028] It should be noted that when extremely hard and wear
resistant surfaces are required, e.g., in some engine components,
it would be a feasible modification of the present invention to
include a plasma nitrided between the substrate and the PVD coating
according to the present invention. Plasma nitriding is an
alternative casehardening process, which is carried out in a glow
discharge in a nitrogen gas-containing mixture at a pressure of
about 100 to about 1000 Pa (about 1 to about 10 mbar). It is one of
the methods used to treat stainless steel surfaces thereby
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. The plasma
nitriding is the most recently developed surface hardening
procedure. This process replaces traditional nitriding methods,
such as gas nitriding and nitrocarburation (short-term gas
nitriding, bath nitriding and tenifer (a salt-bath nitriding
process sometimes called the "Tuffride process") treatment), since
identical thermo-chemical conditions can be established in this
process. Plasma nitriding achieves higher hardness and wear
resistance, while creating lower distortion. Furthermore, palsma
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., are not
necessary.
[0029] The plasma nitriding is performed in a vacuum furnace.
Treatment temperatures in the range of about 400 to about
580.degree. C. are employed, subject to the requirements of the
process in question. Typical treatment temperatures are in the
range of about 420 to about 500.degree. C. Treatment times vary
between about 10 minutes and about 70 hours, depending upon the
component to be treated as well as the desired structure and
thickness of the layer(s) formed. The most commonly used process
gases are ammonia, nitrogen, methane and hydrogen. Oxygen and
carbon dioxide are used in the corrosion-protective step of
post-oxidation. Besides the type of process gas used, pressure,
temperature and time are the main parameters of the treatment
process. By varying these parameters in accordance with the
knowledge of the skilled artisan, the plasma nitriding process can
be fine-tuned to achieve the exact desired properties in any
treated component.
[0030] Any iron-based material can be submitted to plasma
nitriding. The process does not require the use of special types of
nitriding steel. Moreover, the results attained by plasma nitriding
can be reproduced with pinpoint accuracy. This is especially
important in the manufacture of serial products. However, plasma
nitriding does not significantly reduce the static friction. It
would cause no problem to submit the stainless steel to
temperatures in the range of about 450 to about 500.degree. C.
twice, since it will easily resist this temperature without showing
softening tendencies.
[0031] Mechanical Properties of the stainless steel are:
7 Tensile strength, R.sub.m 1700 MPa to 2000 MPa Yield strength,
R.sub.p0.2 1500 MPa to 1800 MPa Elongation 8% to 6% Modulus of
Elasticity 200 000 MPa General hardness 450 to 650 Hv10, about 45
to 58 HRC Surface hardness about 3000 Hv10 Toughness Impact
strength (Charpy V) Min 27 J at -20.degree. C.
[0032] The steel of the present invention maintains its mechanical
properties even after long use at elevated temperatures up to about
400.degree. C.
[0033] The coefficient of thermal expansion of the steel of the
present invention is about 10% lower than that of carbon steel and
more than 30% lower than that of a conventional stainless steel,
such as ASTM type 304L. The steel of the invention is cold formable
and bendable to tight radii. It is also suitable for common
machining operations such as cutting, turning and grinding.
[0034] Further, the steel has good welding properties, when using
TIG and MIG welding methods. Another advantage of the steel of the
present invention is the improved corrosion resistance compared to,
e.g., standard steel ASTM type 304L.
[0035] The principles, preferred embodiments and modes of operation
of the present invention have been described in the foregoing
specification. The invention which is intended to be protected
herein, however, is not to be construed as limited to the
particular forms disclosed, since these are to be regarded as
illustrative rather than restrictive. Variations and changes may be
made by those skilled in the art without departing from the spirit
of the invention.
* * * * *