The Microcrystalline Structure and Performance Behavior of Multiphase Automotive Steels

Of Crystals and Crashes

by Tom Imerito

Since about four decades ago, when Nader’s Raiders, the Environmental Protection Agency and the Arab Oil Embargo converged on the automotive industry, vehicle engineers have been saddled with a set of enigmatic challenges that appear to fly in the face of the basic laws of physics: to build ever lighter vehicles that keep passengers increasingly safe during collisions.

Ron Krupitzer, Vice President, Automotive Applications at the American Iron and Steel Institute (AISI), commented on the formidability of the problem: “Quite frankly the challenges associated with building vehicles that are both crashworthy and low mass are daunting.   The automotive industry is calling on every materials industry to see how efficient we can be.”

However, according to General Motors Technical Fellow, James R. Fekete, PhD., revolutionary solutions remain elusive.  “Nobody has come to us and demonstrated a cost-effective way to make high volume vehicle structures from anything other than steel,” he said.

Newton at Nanoscale

The origin of this conundrum may be found in Isaac Newton’s Laws of Motion which point out two crucial and contradictory facts: 1) lower mass vehicles need less energy to move but; 2) higher mass vehicles absorb more energy in a crash.  One promising solution to the problem of going fast, stopping fast and staying safe has emerged, not at the terrestrial and astronomical scales, usually associated with Sir Isaac’s insights, but at the nano and microcrystalline scales of matter.

In all, today automotive designers can chose from scores of steel types, the strongest of which has about twelve times the strength of the mildest.  U.S. Steel’s Product Technology Division Manager, Joseph Defilippi, enumerated the choices most suitable for automotive frames and body skins: “At the lowest end of the spectrum we have soft, very formable steels.  They go into the outer bodies of automobiles because they’re inexpensive and take a nice finish.  The next family is bake-hardenable steels.  Beyond that, the High Strength Low Alloy (HSLA) steels are good for lightweighting.  Next is dual phase (DP), in which you get favorable work hardening rates, by virtue of their compositions, so you can form more complex parts. Then we go to TRIP (Transformation Induced Plasticity) steels which have even more ductility than dual phase steels and you can make parts that you can’t make out of other steels.  Finally tri-phase steels or complex phase steels mix the best performance features of them all.”

The key to these materials resides in the fact that unlike most materials, steel’s microstructure is amenable to change when in the solid state.  To attempt to untangle steel’s mysterious microstructural behavior: Iron is an element of which steel is an alloy.  When iron is melted with less than one percent carbon, the alloy, carbon-steel is formed.  Iron’s uncommon ability to adopt different elemental structures, or allotropes, combined with carbon’s ability to associate with many elements and to be variously soft, as graphite, and hard, as diamond, make them ideal elemental companions for the synthesis of what is arguably the cheapest, strongest and most plastic material known to man: steel.

The Many Phases of Steel

The synergistic bliss of this elemental kinship is most evident in iron’s collaboration with carbon to establish a common melting and freezing point when heated and cooled in each other’s presence.   That common point, known to metallurgists as the “eutectic point,” also happens to be considerably lower than the melting point of either element on its own.    Alone, iron melts at 1538°C.  Carbon melts at 3527°C.  Together they melt at between about 1170°C and 1450°C, depending on their relative proportions.  What is more, as they cool, they pass through a series of chemical associations and morphological configurations; atoms assuming different electrostatic dispositions in response to heating, cooling and aging; migrating to more stabile neighborhoods; neighbors responding accordingly, all eventually finding a comfortable place within the mass.  Just how they configure themselves determines the crystalline “phase” they will assume and how strong and formable the steel will be.

Like all materials, steel can exist as gas, liquid or solid.  However, steel has the added advantage of having six different solid states or phases.  Steel’s phases are distinguished by the geometric configurations of the microscopic crystals of which they are composed and the temperatures at which those crystals form, reform and transform within the body of a given piece.  Today, these phases are being exploited for their extraordinary performance characteristics.

“We’re starting to take advantage of strengthening sheet steel through phase transformation.”  GM’s Fekete explained.  “We’re heating and cooling steel in a controlled manner to get advanced microstructural components.  With these methods we make steel lighter by making it stronger and using less of it.”

Steel’s principal phases are: Ferrite, a soft phase that forms at two critical temperatures, above 1390ºC and again below 910ºC; Between those temperatures, Austenite, a hard, non-magnetic phase occurs; Cementite, or iron carbide, is a hard chemical compound composed of three iron atoms for every carbon.  Cementite forms in the presence of high carbon concentrations, at between about 400ºC and 1100ºC; Pearlite, a two-phase mixture that forms in the same temperature range as cementite, but at lower carbon ratios, is made up of alternating layers of soft ferrite and hard cementite; Martensite is a super-hard phase that often occurs as a consequence of the very rapid quenching of austenite; Bainite, a strong, ductile steel alloy, compositionally similar to pearlite, forms when austenite is cooled more quickly than necessary to form pearlite, but not quickly enough to form martensite.

According to Professor John Speer of the Colorado School of Mines, steel is exemplary among the metals for its wide array of phases and combinations thereof.  Speer says that alloying, work hardening and thermal control are the three historic and enduring variables of steel processing.  Alloying refers to iron’s proclivity for engaging in crystalline synergies with other metals to form metals with exceptional qualities.  Work hardening refers to steel’s tendency to become stronger, though less ductile, when subjected to mechanical force, such as a hammer blow, a rolling mill or a forging press.  Thermal control refers to steel’s ability to be made variously harder by rapid quenching or moderate baking, or softer by long, slow heating, or annealing, while in the solid state.  In its own way, each of these processes influences the performance characteristics of a piece of steel.   Although chemistry and mechanical force are important to steel manufacture, clearly, heating and cooling play essential roles in phase transformation.

The Magic Is in the Crystals

As liquid iron and carbon cool and solidify, the loss of thermal energy allows the previously freewheeling atoms to adopt preferred spacing arrangements between and among each other.  In hard materials throughout the natural world, these spacing preferences are exhibited in the form of crystals.  Crystals are one of Mother Nature’s ways of keeping atoms happy, that is, at their lowest energy level. Crystals are formed as a consequence of the dynamic electrostatic attraction/repulsion of atoms in close proximity to each other.

The smallest and most basic crystalline form is called a “unit cell.”  Unit cells measure roughly one half of a nanometer or billionths of a meter in length, depending on the dimension in question and the exact crystalline phase.  A unit cell of basic iron, or ferrite, is represented graphically as a “body-centered cubic,” configuration, with one atom at each corner and one dead-center in the middle of a cube, for a total of nine atoms.  As ferrite crystals cool, they transform themselves into a sequence of different configurations by taking on new iron atoms and rearranging themselves into various sizes, shapes and densities, each with the same elemental ingredients, but in different proportions and with different properties.  Each of these configurations is a different phase.

Once crystal formation has commenced, the initial nanoscale crystal unit takes on more cooling atoms, extending its three-dimensional configuration.  The upper left atom on the face of the first unit cell becomes the lower right atom on its neighbor, and so on.  This process continues, forming an increasingly large crystal lattice, until something stops it from growing.  A reduction in heat or a gentle bump from a neighboring lattice, or “grain,” typically serves to inhibit further crystal growth.  Grains are normally composed of many thousands of unit cells.

Welcome Imperfections

As neat as this system of crystals, lattices and grains sounds at first, it is important to know that steel’s uncommon combination of strength and ductility comes not from perfect lattices, but rather from the imperfections they contain.  Because atoms of alloy metals are always different in size and electrical charge than iron, they cause imperfections in the otherwise symmetrical geometry of the lattices in which they reside.  These imperfections come in two forms: strains and dislocations.  Generally speaking, strains make steel stronger while dislocations make it more formable.  Strains distort the geometry of individual unit cells, forcing them to exert force on neighboring cells thereby making the lattice stronger.  Dislocations are mobile faults between rows of unit cells within the lattice.  Because lattice dislocations can move, they allow the lattice to deform.  Somewhat curiously, because strains distort the symmetry of lattices, they tend to impede the movement of dislocations.   Because lattice strains can occur as a consequence of a mechanical force, such as a collision, dislocation mobility is diminished with each subsequent nanoscale dislocation event.  As such, as a piece of steel collapses under mechanical force it becomes increasingly resistant to further dislocation movement every time it moves.   Resistance to dislocation movement results in reduced ductility hence increased strength, a desirable crash-performance trait.  Even more curious is that faster deformation results in more dislocations hence increasingly stronger steel – another desirable crash-performance characteristic.  Metallurgists refer to this phenomenon as strain rate hardening, one of the most counter-intuitive of steel’s many behavioral curiosities.

The Key to Collision Performance

The optimization of steel to achieve superior crash performance is achieved in several ways: 1) By the control of grain size which is achieved by alloying and thermal control – generally, smaller grains result in stronger steel; 2) By the orientation of grains across the mass by rolling and forging and; 3) By commingling variously ductile, strong and tough phases by precise thermal control during heating and cooling.

Putting the Key in the Ignition

Today, anticipated increases in side impact and roof crash requirements are leading to explorations and tests of high tensile strength multi phase steels for increased passenger safety, including martensite-containing dual phase steels which can be up to five times as strong as traditionally employed steels.  General Motor’s Fekete, who expects a ten-fold increase in the use of dual-phase steel by the year 2010, pointed to a study showing that substituting martensite-containing dual phase (DP) steel in place of traditional high strength low alloy carbon steel (HSLA) could yield at least a ten-percent reduction in vehicle mass, without compromising energy absorption.  Fully martensitic steels, traditionally used in bumper and door beams, have recently been used for roll formed rocker panel reinforcements in the Cadillac CTS/STR/STS.   Although the performance advantages are obviously high, the difference in manufacturing cost between a roll formed mild steel part and the same part in martensitic steel is negligible.  In addition, highly formable TRIP steels are beginning to find applications in GM’s Opel Astra.

What Newton Didn’t Tell Us

As a practical matter, during a collision, multiple phase steels essentially increase the sheer number of nanoscale energy-absorbing events by spreading the bulk crash energy over billions of dislocation sites in rapid succession within the body of the steel bumpers, beams, columns and rails that comprise a vehicle’s body:  More nanoscale energy absorption with less bulk-scale mass.  Maybe Sir Isaac knew more than we think.

This story originally appeared in the Society of Automotive Engineers SAE International magazine. To purchase a copy from their website, click here.

©Copyright 2007 Thomas P. Imerito/ dba Science Communications