ENVIRONMENTAL ELEMENTS: HOW THEY CAN INFLUENCE THE SPRINGS’ LIFETIME

ENVIRONMENTAL ELEMENTS: HOW THEY CAN INFLUENCE THE SPRINGS’ LIFETIME

In our journey through the factors that influence spring breakage, we have already carefully examined some crucial aspects:

WHY DOES A SPRING BREAK

DIFFERENCES BETWEEN STATIC STRESS AND DYNAMIC STRESS

FATIGUE FAILURE OF A SPRING: WÖHLER AND GOODMAN DIAGRAMS

SPRING MANUFACTURING : WHICH MATERIALS ARE USED

THE MATERIAL STRUCTURE: HOW IT AFFECTS THE QUALITY OF SPRINGS

THE SURFACE CONDITION OF WIRE: ANOMALIES THAT CAN DAMAGE SPRINGS

SHOT PEENING: HOW DOES IT IMPROVE THE FATIGUE RESISTANCE OF SPRINGS

CONTROLLED SHOT PEENING: METHODS AND PRECISION

In this new article, we will focus on another important element: the environmental impact on the springs’ performance.

The importance of the environment

We already discussed about materials, mechanical stresses and manufacturing processes. Now it’s time to explore how the context in which springs are used can influence their durability and reliability. We will analyze environmental elements, operating conditions and risks associated with the use of springs.

Whether they are industrial, automotive or other applications, understanding the interaction between springs and the surrounding environment is essential to designing safe and efficient systems.

Environmental factors and their impact

Several applications of the springs

It is known that springs are used in many different sectors; in fact, any time it is necessary to store mechanical (elastic) energy and then return it, the spring finds its use.

And this happens everywhere, both outdoors and indoors, in outer space, in the engine compartment of a car or in the hydraulic circuit of a household appliance.

The importance of the usage context

The first question the designer of elastic components must ask is where the spring is used. The context defines the first choices, in particular on materials and just after on manufacturing processes.

The two most important environmental factors are:

• aggressiveness in terms of corrosive agents;

temperature range of use.

Corrosion: an invisible enemy

Explaining the corrosive process

Corrosion of metals is an electrochemical process, where on the metal surface occur some oxidation-reduction reactions – as in a galvanic cell – with a chemical form that oxidizes (Anode, loses electrons from its outermost layer) and another chemical form that reduces (Cathode, acquires electrons). In the case of alloy and non-alloy steels, which are mainly made up of iron, the form that oxidizes is iron, while the one that is reduced is the oxygen contained in the water. caso degli acciai legati e non legati, che sono prevalentemente costituiti da ferro, la specie che si ossida è ovviamente il ferro, mentre quella che si riduce è l’ossigeno contenuto nell’acqua.

Iron oxide or, better, ferric oxide (Fe 2 O 3 ) – the common red rust – has two properties that make it less than desirable: it has mechanical properties that are much lower to those of pure iron and tends to detach, therefore reducing the volume of the metal object that it leaves.

This double action has a ” structural ” effect on metal components, including springs. If the resistant section decreases with the same applied load, on the other side the stress increases, and if this first exceeds the yield load and then the breaking load, first occurs mechanical failure and then ductile fracture .

In a nutshell: the wire or the strip with which the spring is made breaks. Corrosion cracking is often a trigger point for failure due to the fatigue mechanism. It is understood that structural continuity is a prerequisite to avoid fatigue failures and corrosion this principle.

Environments with high risk of corrosion

Outdoor environments are corrosive due to the hot-cold cycles between night and day, which lead to the formation of humidity, which triggers surface corrosion.

The presence of chemical agents (for example chlorides) which induces the formation of acids is worsening from the corrosion point of view. This means that the following contexts are critical for springs:

– Roads where salt is used in winter.

– Marine environments both near the sea and offshore.

– Industrial environments that work in presence of vapors with acid/basic agents.

Corrosion resistant materials

The evaluation of the usage environment is the driver is the driver in choosing the material with which to manufacture the springs. In the corrosion resistance scale – ordering the materials from least resistant to most resistant – we find:

• Carbon steels (work hardened) / alloy steels ( SiCr and more).

• Stainless steels with austenic structure (1.4310 or 302).

• Austenic stainless steels with Molybdenum (1.4401 or 316).

• Austenitic steels with high nickel-chromium content (Duplex 1.44621, 4539-T904L .. ).

• Nickel alloys ( Inconel , Nimonic , Monel), nickel-cobalt ( Phynox , Hastelloy) and titanium alloys.

The latter are almost immune to corrosion, but of course the costs are higher.

Protezione dalla corrosione: strategie e materiali

Corrosion protection strategies

Since not all sectors can afford the costs of special alloys, was created the corrosion protection sector, which with various processes ensures “poor” steels the opportunity of being used in medium aggressive environments.

The idea is to prevent contact with the agents that trigger the corrosive process by placing a protective layer between the metal and the environment. The performance of the protection depends on the protective effectiveness of the anti-corrosion layer. In fact, any action that interrupts the continuity of the protective layer exposes the metal to contact with oxygen and facilitates corrosion. There are also ionic diffusion processes, where the corrosive agents overcome the protective layer without damaging it and act underneath it.

Most used protection systems

Herunder some of the most used protection systems:

• Oiling/greasing.

• Phosphating, often used as a pre-treatment for painting and Teflon coating.

• Epoxy (powder) and cataphoresis painting.

• Teflon coating, which has also other scopes than protection, in this specific case the reduction of sliding friction..

• Galvanic protections with some different more or less precious metals. Including galvanizing, chrome plating, nickel plating, tin plating, copper plating, rhodium plating, silver plating, gold plating.

For springs in work-hardened carbon steel, galvanizing is widely used. But pay attention, not all galvanic treatments can be used on all kinds of springs. For example, for pre -hardened alloy steels ( SiCr , SiCrV .. ), it would be better to avoid galvanic processes, especially those that use an acid pickling bath. The problem is called hydrogen embrittlement (Hydrogen embrittlement): Hydrogen enters into the granular matrix of the steel and makes cracks that become increasingly deeper during alternate use of the springs and lead to premature failure, due to fatigue.

• Painting with flaked zinc, Geomet 320, Zintek 200, Magni 565, Deltatone , Deltaprotekt with various additions (PFE or other).

• Zinc-Nickel.

Therefore the environment has an initial strong impact on the design choices relating to the material to be used to manufacture the springs. From this first choice derive then the oter ones, because the material brings with it mechanical performances (Re and Rm) to deal with and which define the geometry of the spring.

The effect of temperatures on spring performance

The mechanical performance of steels remains more or less unchanged in a defined temperature range. This performance is limited at both low and high temperatures.

Let’s start by separating the two extreme fields.

Impact of low temperatures

Work-hardened carbon steels work without problems up to 20° C, alloy steels ( SiCr ) down to -30° C. Below these temperatures embrittlement is observed which can lead to breakages.

Therefore, in presence of thermally adverse environments – areas of the extreme North or South, industrial applications linked to cryogenics and above all aerospace, where temperatures drop to levels not reachable on earth – stainless steel and nickel alloys , remain the only solution. Stainless steel 1.4310 can reach up to -150°/-200° C.

In the aerospace sector, the use of nickel and titanium alloys is preferred, not only for their stability at low temperatures, but also for their performance at high temperatures.

Impact of high temperatures

Work-hardened carbon steel shows signs of failure under load already at temperatures above 100°C.

High temperatures mainly have a relaxation effect. The temperature leads to a dislocation of the structural matrix of the steel, which stretches with a permanent effect. In the case of the compression spring, this means a shortening of the spring and therefore a lower force at the same height .

This effect is effectively rendered with the failure percentage . The difference in load between before and after relaxation can be calculated and compared to the initial load, percentageizing the value obtained. The failure phenomenon has a logarithmic evolution, that is a failure pattern repeated for decades.

Excluding the initial phase (72-96 hours) which has a high failure, in the subsequent time the failure repeats for every decade. That is, if between 100 and 1,000 hours the spring gives 2%, it will also be the same between 1,000 and 10,000 hours. Below you can see a relaxation curve between 0 and 10,000 hours.

environmental factors - spring failure graph

The pressure drop due to relaxation can be reduced by carrying out block settlement processes, both at room temperature (cold blocking) and at heat (hot blocking).

Strategies for managing temperature variations

In the latter case there are different techniques to induce failure, and all of them work in the first part of the area of the graph above; they work to obtain a preventive controlled failure , which leaves only the residual part of the failure to the long-term process.

Pre -hardened alloy steels ( Si-Cr ) are effectively used in this area, with temperatures up to 130°C, especially in internal combustion engine valves. Hot settlement, combined with the structural characteristics of the material, guarantee limited failure in service, even in relatively hot environments.

For medium-high temperatures 120-200° C, austenitic stainless steels are used.

For high temperatures, nickel alloys ( Inconel , Nimonic , Monel ) are used, up to a maximum of 700° C and nickel-cobalt alloys up to 1,000° C.

Beyond this limit there are no other alloys with elastic uses that can support us.

Environmental factors: how they are managed at Mollificio Valli

At Mollificio Valli the environmental aspects linked to the use of springs are taken into due consideration, starting from the design phase. We always suggest to the customers to provide a broad and detailed description the environmental usage of the spring, as well as the performances considered.

Corrosion protection processes are well known and often used , working together with our suppliers operating in this sector. By knowing in detail limits and advantages of the various corrosion protection methods, we always support our customers, in order to combine both the performance and economic aspects.